a more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings . fig1 a general description is made for a copy machine according to an embodiment of this invention . a charging roller ( member ) 11 is set in contact with a photosensitive body 12 , applies a voltage from a voltage applying device 15 to charge a surface of the above photosensitive body 12 uniformly . the photosensitive body 12 , shown is in a drum form ( although belts and other media are contemplated ) rotates in a clockwise direction . it is known that components are arranged to execute an electrophotography process around the photosensitive body 12 in its rotating direction including the above charging roller 11 , an exposing portion , a developing unit , a transfer / separation unit to a copying paper , a cleaning unit , and a charge removing unit in this order . the voltage applying device 15 applies a voltage to the charging roller 11 under control of the control means 16 . a temperature detection device 13 detects a temperature of the charging roller 11 , wherein the charging apparatus is a contact - type charging apparatus in which the voltage applied to the charging roller is compensated in accordance with the detection temperature detected by the temperature detection device 13 . the charging apparatus further includes a potential sensor 14 for measuring a charging potential of the photosensitive body , wherein the charging potential created by applying voltage at predetermined one or plural points is detected . a voltage to be applied required for making the charging potential of the photosensitive body equal to a target potential is obtained based on the detected charging potential , and the difference between the obtained voltage to be applied and a compensated applied voltage based on the detected temperature at that time is detected . the charging apparatus is also provided with a compensation mode of compensating a compensation rule of the applied voltage based on the detection temperature in accordance with the detected difference . the above charging roller 11 includes , for example , an epichlorohydrin rubber roller or a roller having a coating film on its surface made of fluoroplastic with hydrin rubber and silica dispersed thereon . the charging characteristics depend on the temperature of the roller as shown in fig7 and also depend on the humidity as shown in fig8 . since a change amount of the charging characteristics caused by the temperature is several times larger than a change amount caused by the humidity , it is possible to control the charging stably without exerting any influence upon the charging potential due to the temperature variation or the humidity variation , by switching the compensation for the detected temperature per each humidity area . in this embodiment , a voltage applied to the charging roller 11 is compensated based on a rule shown in fig2 in accordance with the temperature detected by a charging roller temperature detection device 13 . when operating in a compensation mode , with reference to the flowchart of fig4 in step s1 charging potentials vs1 and vs2 are detected on the photosensitive body corresponding to applied voltages vr1 and vr2 (- 1 , 000 v and - 1 , 500 v , respectively , in this embodiment ) are determined at different two points previously determined by a photosensitive body potential sensor 14 disposed in a lower stream from the charging roller 11 in a rotating direction of the photosensitive body 12 . since an applied voltage vr0 required for making a charging potential equal to a target potential vs0 (- 900 v , in this embodiment ) has a relationship between the photosensitive body charging potentials vs1 , vs2 and the applied voltages vr1 , vr2 as shown in fig3 the applied voltage vr0 is obtained , in step s3 , by the following equation : ## equ1 ## in this embodiment , an error ( deviation ) of the charging potential caused by a humidity variation is compensated by obtaining , in step s5 , a difference vrs between an applied voltage vr ( t ) compensated by a charging roller detection temperature t at that time and vr0 , and by performing the charging operation in the compensation mode and thereafter with the applied voltage obtained by adding in step s7 the difference vrs to the above applied voltage thus compensated . in addition , as shown in fig5 the charging potential can be controlled more accurately by selecting a compensation rule according to the difference vrs out of a plurality of compensation rules in accordance with a plurality of humidity regions which have been prepared and then compensating the applied voltage based on the selected rule to execute the subsequent charging . the process for performing the above procedures is shown with reference to fig6 . the process begins in step s11 where vs1 and vs2 are detected on the photosensitive body 12 . the process proceeds to step s13 where equation 1 ( as set forth above ) is calculated to obtain vr0 . the process then proceeds to step s15 where vrs is calculated as was done in step s5 in fig4 . the process then proceeds to step s17 which inquires whether vrs is greater than or equal to 0 . if the answer is affirmative , the process flows to step s19 where it is determined if the absolute value of vrs is greater than one - half of the difference between an absolute value of vr ( t ) and vr &# 34 ;( t ). if the result of inquiry in step s21 is affirmative , the temperature is compensated with vr &# 34 ;( t ), but if the result is negative , the temperature is compensated with vr ( t ). if the result in step s17 is negative , the process flows to step s25 where it is determined if the absolute value of vrs is less than or equal to one - half of the difference between vr &# 39 ;( t ) and vr ( t ). if the result of the inquiry in step s25 is affirmative , the process flows to step s23 where the temperature is compensated with vr ( t ). however , if the result in step s25 is negative , the process flows to step s27 where the temperature is compensated with vr &# 39 ;( t ). fig1 shows a main portion of an image forming mechanism of a second embodiment according to the present invention . a drum - shaped photosensitive body 101 has a conductor substrate layer made of aluminum or the like and an optical conductor layer formed on its outer peripheral surface as a basic construction layer . a charging roller 102 is brought into contact with , and charges a surface of , the photosensitive body so to charge the photosensitive body 101 to a predetermined polarity and potential . fig1 shows a supporting structure for the charging roller 102 . the charging roller 102 includes a core metal , a conductive layer formed on its outer peripheral surface , and a resistive layer formed on its further outer peripheral surface . two axes 121 arranged at both ends of the core metal are supported so as to rotate freely at each of an arm 124 and two bearing blocks 122 are in parallel with a photosensitive body 101 . each of the bearing blocks 122 is guided so as to reciprocate freely in a radial direction of the photosensitive body 101 to end plates ( not shown ) at both ends of the supporting frame 128 and pulled by ( two ) tensile coil springs 127 in a direction that it is separated from the photosensitive body 101 . the arm 124 is permitted to rotate freely around an axis 123 . the arm 124 is connected to an end of a tensile coil spring 126 and the other end of the spring 126 is connected to a rod 125a of a contact / separation driving solenoid 125 . when the solenoid 125 is energized , the rod 125a is pulled upward and the arm 124 rotates in a clockwise direction , so that the charging roller 102 is brought into contact with the photosensitive body 101 ( shown by two - dots - and - dash line ). it is a position of application for charging the photosensitive body 101 . a contact pressure corresponds to a difference of a tension between tensile coil springs 126 and 127 . once the solenoid 125 ceases to be energized , the arm 124 rotates in a counterclockwise direction and the charging roller 102 is separated from the photosensitive body 101 so as to be brought into contact with a cleaning pad 120 ( escape position ). a gear , which is firmly fixed on the core metal of the charging roller 102 , is engaged with a driving gear 129 at the escape position . when the driving gear 129 rotates , the charging roller 102 also rotates and its surface rubs against a pad 120 , so that toner ( dirt ) on the surface thereof is wiped out by the pad 120 . when the charging roller 102 is in contact with the photosensitive body 101 by energizing the solenoid 125 ( position indicated by two - dots - and - dash lines in fig1 ), the charging roller 102 rotates following a rotation of the photosensitive body 101 . when the charging power supply circuit 130 ( fig1 ) applies a charging voltage to the core metal ( 121 ) through the tensile coil spring 127 and a bearing block 122 , the peripheral surface of the photosensitive body 101 is uniformly charged . referring again to fig1 , an exposing apparatus 108 exposes the surface of the photosensitive body charged by the charging roller 102 with an image light , and thereby an electrostatic latent image is formed thereon . for an exposure of areas other than the area to be transferred to a transfer paper ( image area ) on the surface of the photosensitive body , an eraser ( light emitting element group ) 109 is used , which makes a potential at a toner - unattached level . toner is applied to the electrostatic latent image in the image area by a developer 110 , which makes a toner image ( visible image ) appear on the electrostatic latent image . in the fourth embodiment , the exposing apparatus 108 lights a manuscript document on a manuscript document stand ( contact glass ) with an exposing lamp to project a reflection light from the manuscript document to the photosensitive body 101 through a mirror and a lens . the above toner image formed on the photosensitive body 101 is transferred to a transfer paper which is fed onto a transfer belt 111 so as to be synchronized with a movement of the toner image . in other words , a potential for absorbing toner onto the transfer paper is applied to a rear side of the transfer belt 111 and thereby the toner image is transferred to the transfer paper . the surface of the photosensitive body after the transfer operation is wiped out by a cleaning blade of a cleaning apparatus 113 , which removes toner remaining on the surface of the photosensitive body . further , charges on the surface of the photosensitive body are removed by receiving a light irradiation from a charge removing lamp 114 and it is shifted to the charging roller 102 . between the charging roller 102 and the developer 110 , a surface potential sensor 105 detects a surface potential vch after charging the photosensitive 101 . a p sensor 112 detects a toner density on the surface of the photosensitive body 101 . these sensors are used in controlling a charging potential , a toner density , or a voltage of the exposing lamp , all described later . when the charging roller 102 completes a charging process ( applying a voltage for charging ) for forming an image , the roller is separated from the photosensitive body 101 due to interruption of energizing the solenoid 125 in the contact / separation mechanism 104 in fig1 and returns to the escape position shown by a solid line in fig1 . then , when it remains in the escape position and predetermined cleaning start conditions are satisfied , the driving gear 129 is driven to rotate , the charging roller 102 rotates , and a surface of the roller is cleaned by the pad 120 . fig1 outlines a configuration of a control system of the image forming mechanism of fig1 . first , there is provided a control section having a microcomputer which comprises a cpu 160 , a ram 161 , a rom 162 , an eeprom 167 ( a nonvolatile storage ), and input / output port buffer amplifiers 163 and 164 , and the control section controls an automatic document feeder ( adf ) 180 and the exposing apparatus 108 by performing serial communication between a txd , rxd , and pc2 terminals in the cpu 160 . in this serial communication , the adf 180 communicates with the above control section when the output of the pc2 is at a &# 34 ; high &# 34 ; level , while the exposing apparatus 108 communicates with the control section when the output of pc2 is at &# 34 ; low &# 34 ; level . a microcomputer ( not shown ) in the adf 180 performs paper feeding / discharging processing and detects a jam for the manuscript document , based on data transmitted from the control section of a copying machine . on the other hand , a microcomputer ( not shown ) of the exposing apparatus 108 controls driving of a scanner or a mirror based on the data transmitted from the control section . the cpu 160 contains a recording paper selecting means , a recording paper reusing means , a defective print preventing means , and conveying restarting means as firmware . a pulse generator 165 generates a synchronous pulse per rotation through a minute angle in synchronization with a rotation of the photosensitive drum 101 , and the above control section controls feeding transfer paper and feeding a manuscript document and performs image forming processing ( particularly , a timing control ) based on a count value of the pulses generated by the synchronous pulse generator 165 . the synchronous pulse is generated by the pulse generator 165 in synchronization with a rotation of the photosensitive drum 101 and then given to the cpu 160 . the cpu 160 increments a count of the arriving pulses by executing an interrupt handling process whenever a pulse has arrived , and if the count value matches any count value on a timing table ( a table in which a relationship between count values and events are stored ) when it is compared with count values on the timing table , the cpu executes an event ( on / off of an image forming element ) assigned to the count value . fig1 outlines a control process performed by the cpu 160 . if a power supply is turned on , the cpu 160 sets an internal register , a counter , and a timer to values in a standby state and sets an input / output ports for a facility ( mechanism ) unit to signal levels at standby state ( step 101 ). after the initialization ( step 101 ) is finished , the cpu 160 reads a state of the facility unit and checks whether or not an error occurs ( a state in which an image cannot be started to be formed ) ( steps 102 and 103 ). if an error is detected , the cpu displays it on an operation board 166 ( step 104 ). otherwise , it starts energizing a heater of a fixing apparatus , sets a target temperature to a value at a standby state , starts a warming - up to the target temperature , and then checks whether or not a fixing temperature ( a temperature of a fixing roller of the fixing apparatus which is not shown ) is set to a standby temperature . if it is not set to the standby temperature , the cpu waits until it is set to the standby temperature . when it is set to the standby temperature , the cpu displays ready ( image forming possible ) on an operation displaying portion and reads a display of an operator manipulation on the operation board 166 , if it is found on the operation board ( step 105 ). at this time , the cpu writes inputs such as the number of pieces of recorded forms , a recording magnification , and a recording density into a register , if there are any inputs . hereupon , the register signifies a memory area allocated to the internal memory of the cpu 160 or the ram 161 or the eeprom 167 . the process then flows to step 106 , where an inquiry is made whether the process is to start . if so , the process flows to step 107 and the cpu 160 displays the fact on the operation board 166 , updates a target temperature of the fixing apparatus 108 to a higher temperature for fixing processing ( responding to it , a driver for controlling a fixing temperature switches an energizing current for a fixing heater to a higher level ), and then starts driving the rotation of the photosensitive drum 101 and turning on a charge removing lamp ( charge removing exposure ). then , predetermined voltages vap1 , vap2 , and vap3 are applied to the charging roller 102 sequentially each for a fixed time period , and then the values vch1 , vch2 , and vch3 detected by the surface potential sensor 105 are converted to digital data to be read at a timing when areas on the photosensitive body charged at respective voltages come immediately before the surface potential sensor 105 ( step 107 ). next , in step 108 the cpu 160 obtains a linear equation which represents the relationships among three points , ( vap1 , vch1 ), ( vap2 , vch2 ), and ( vap3 , vch3 ) most accurately . in other words , assuming that this linear equation is vap = a · vch + b , coefficients a and b at which differences between the above three points and the linear equation are minimum are obtained in a method of least squares to determine a linear equation , vap = a · vch + b which represents the relationships between a voltage vap applied to the charging roller 102 and a charging potential vch on the photosensitive body 101 caused by the voltage . after that , data representing the linear expression is saved in the register ( step 108 ). fig1 shows a transition of a correlation of a charging potential vch of a photosensitive body to a voltage vap applied to the charging roller ( characteristics of forming a charging potential ) according to an elapsed time ( the number of times for forming an image ) for using the photosensitive body 101 and the charging roller 102 . this correlation is expressed substantially by a straight line . therefore , as described above , assuming that the correlation is expressed by vap = a · vch + b in this embodiment , this straight line is obtained ( calculated ) based on actually measured values ( vap1 , vch1 ), ( vap2 , vch2 ), and ( vap3 , vch3 ). in other words , characteristics of forming the current charging potential ( correlation ) are judged ( determined ) ( step 108 ). referring again to fig1 , the cpu 160 in step 109 sets a start / stop timing of charging , exposing , erasing , feeding , developing , and transferring , etc . for forming an image ( recording a piece of an image copy ) in the timing table according to an already - entered recording mode . if no recording mode is entered , the &# 34 ; standard mode &# 34 ; is selected . if no parameter is entered , the &# 34 ; standard value &# 34 ; is selected . the voltage vap applied to charging roller 102 is obtained by setting vap = vapp which is obtained by giving vch = vchp ( target charging potential ) to the above determined straight line , and determining vap = a · vch + b . also in step 109 a single copy cycle ( a single image forming processing ) is performed to increment a recorded copy counter ( register ) by one . in this single copy cycle ( step 109 ), the cpu 160 executes process controls of charging ( by using the charging roller 102 ), exposing , developing , and transferring , and in this charging process by use of the charging roller 102 , the above vapp is used for the charging voltage applied to the charging roller 102 . in other words , assuming that vapp is a target value of a voltage applied to the charging roller 102 by a charging power supply circuit 130 , an output voltage of the power supply circuit is applied to a controlling driver . monitoring ( feeding - back ) a voltage applied to the charging roller 102 , the driver performs a constant voltage control for the power supply circuit 130 so that the applied voltage matches the target value vapp . next , in step 110 , the cpu checks whether or not the number of copied sheets ( number of times that an image is formed : number of continuous copied sheets ) has reached the set copy count . if the number of sheets does not reach the set count , the cpu executes a single copy cycle ( step 109 ) again . if it reaches the set count , the cpu returns the target temperature of the fixing apparatus to a value at standby state , sets postprocessing ( end cycle ) such as ( continuous time for ) cleaning of the photosensitive body , the transfer belt , and the charging roller ( step 111 ), and awaits the arrival of an input to the operation board 166 ( step 105 ). if the end cycle terminates without receiving any start input from the operation board 166 , the cpu stops the rotation driving of the photosensitive drum 101 and turns off the charging lamp to stop the end cycle ( steps 112 and 113 ). in other words , the facility unit is put into a standby ( stop ) state . fig1 shows a timing diagram of elements related to image forming during the time period from a start input to termination of an end cycle ( steps 106 to 113 ) in the above . this drawing shows the case of performing an operation for two copied sheets specified . in this operation , a pre - rotation period is started by starting a rotation of the photosensitive body 101 of an apparatus which has been put in a standby state on the basis of a print starting signal ( start input ). the charge removing lamp 114 is turned on at the same time when the rotation of the photosensitive body 101 is started , and charges are removed on one peripheral surface or wider range of the photosensitive body 101 . next , three predetermined voltages vap1 , vap2 , and vap3 are applied to the respective predetermined - wide areas on the charging roller 102 sequentially . a surface potential sensor 105 detects the surface potentials vch1 , vch2 , and vch3 of the photosensitive body at the time of applying the respective voltages . the relationships between the applied voltages vap1 , vap2 , and vap3 and the charging potentials vch1 , vch2 , and vch3 are recurred to a straight line by the cpu 160 to obtain a correlation of the charging potential vch to the applied voltage vap . an applied voltage vapp at the time of the subsequent image forming is set based on the correlation . printing ( image forming ) for the first sheet is explained now . the above voltage is applied to the charging roller 102 to charge the photosensitive body 101 , the exposing apparatus 108 exposes a charged surface with an image light to form a electrostatic latent image thereon , the image is developed by the developing apparatus 110 , transferred to a transfer material through a transfer process , and thereafter fixed by the fixing apparatus which is not shown , and then the sheet is output . in the same manner as for continuous printing , the applied voltage at the time of forming the respective images is controlled to the above vapp constantly . in the above configuration and the operations thereof , even if a resistance of the charging roller 102 as a charging member changes due to the effects of environmental conditions ( for example , humidity or the like ), the above detected potentials vch1 , vch2 , and vch3 are lowered if the resistance value goes up , and the applied voltage vapp is determined based on the potentials to form an image . therefore , the charging potential of the photosensitive body 101 is fixed to a target value independently from changes of the resistance values of the charging roller 102 . a film thickness of the photosensitive body 101 may be decreased in some cases since the surface of the photosensitive body 101 is abraded by being rubbed against the cleaning apparatus 113 or a transfer belt 111 , and a current which flows through the photosensitive body 101 is increased by a decrease of the film thickness at this elapsed time and it leads to a problem that the charging potential is lowered at the elapsed time when a constant current control is performed . in addition , even if the film thickness does not change , a continuous use of the photosensitive body causes electrostatic fatigue and values of the current flowing through the photosensitive body are different between the states after it has been left for a long time and after it has been continuously used . the above matters lead to a problem that the charging potential depends on a state of the use if a constant current control is performed . this embodiment , however , is effective for solving this problem . furthermore , since it is impossible to prevent the resistance value from being uneven at the time of manufacturing the photosensitive body 101 and the charging roller 102 , an applied voltage must be adjusted to obtain an optimum surface potential for each machine . this problem can be also solved by executing this embodiment in which the voltage can be easily adjusted . although a hardware configuration of a third embodiment is the same as for the second embodiment in the above , a part of the image forming control of the cpu 160 is not identical . fig1 shows an outline of a control operation of the cpu 160 in the third embodiment . for steps whose processing is the same that shown in fig1 , the same step number of fig1 are used for fig1 . in this third embodiment , the cpu 160 applies three voltages vap1 , vap2 , and vap3 predetermined at the three points to a charging roller 102 sequentially for respective time periods responding to the first start input after the power supply is turned on in the same manner as that of the second embodiment ( fig1 ), and then converts the detection values , vch1 , vch2 , and vch3 detected by the surface potential sensor 105 to digital data to be read , at a timing when areas of the photosensitive body charged at respective voltages come immediately before the surface potential sensor 105 ( step 107a ). then , a linear equation vap = a · vch + b is determined , which represents a relationship between a voltage vap applied to the charging roller 102 and a charging potential vch generated by the voltage on the photosensitive body 101 ( step 106a ). next , in step 122a a target applied voltage vap = vapp is calculated by giving vch = vchp ( target charging potential ) to the determined linear equation vap = a · vch + b , the photosensitive body 101 is charged by giving the value thus calculated to a driver for controlling an output voltage of a charging power supply circuit 130 to a constant voltage as a target value , and an optical pattern ( exposed / unexposed pattern ) for detecting a toner density is projected on the charging surface to detect toner densities ( vsg for an exposed area , vsp for unexposed area ) of areas ( an exposed area and an unexposed area ) developed by the developing apparatus 110 by use of a p sensor 112 . next , after calculating a toner supplying amount ( a rotational driving time for a toner supplying roller 118 ) corresponding to a ratio vap / vsg of the toner density vsp for the unexposed area ( a black written area ) to the toner density vsg for the exposed area ( a background area ) ( step 123a ) if the value exceeds zero , the cpu starts the rotational driving of the toner supplying roller 118 and measurement of the elapsed time ( step 124a ). if the elapsed time has reached the above calculated driving time , the cpu stops the rotational driving of the toner supplying roller 118 . after that , a single copy cycle is repeated ( steps 109 , 110 ) until the number of copied sheets reaches a set copy count in the same manner as that of the above second embodiment and processing proceeds to an end cycle . in the third embodiment , however , when the end cycle is set , the cpu 160 executes again the same processing as the above &# 34 ; detecting the charging potential vch &# 34 ; ( step 107a ), &# 34 ; setting applied voltage vap &# 34 ; ( step 108a ), &# 34 ; detecting a pattern density &# 34 ; ( step 122a ), &# 34 ; calculating a toner supplying time &# 34 ; ( step 123a ), and &# 34 ; starting toner supplying &# 34 ; ( step 124a ) in the same manner ( steps 107b , 108b , 122b , 123b , and 124b ). then , the cpu writes data , &# 34 ; 1 : a charging voltage is set and the toner density is adjusted immediately after the termination of the copy .&# 34 ; other processing is the same as that of the above second embodiment . when any start input is given next without turning off the power supply of the apparatus , the above steps 107a , 108a , 122a , 123a , and 124a such as &# 34 ; detecting a charging potential vch &# 34 ; immediately after the above start input are not executed since the data in a register rif are indicated by 1 , and the steps 107a , 108a , 122a , 123a , and 124a such as &# 34 ; detecting a charging potential vch &# 34 ; are executed immediately after the termination of the set copy count . fig1 shows an operation timing of elements related to image forming from a start input immediately after turning on the power supply to a termination of an end cycle ( steps 106 to 113 ) of the third embodiment . it is intended for 2 sheets to be specified as the number of copied sheets . a pre - rotation period is started by a rotation of the photosensitive body 101 based on the first print start signal ( start input ) after the power supply is turned on . at the same time when the rotation of the photosensitive body 101 is started , the charge removing lamp 114 is turned on and charges on the photosensitive body 101 are removed by one peripheral surface or wider range . next , a relationship between an applied voltage vap and a charging potential vch generated by the voltage is detected . after a charging voltage vap calculated based on this relationship is applied to the charging roller 102 to charge the surface of the photosensitive body 101 , charges are removed by an eraser 109 for areas other than a range of 65 mm × 35 mm read by a p sensor 112 , in other words , a toner pattern area , and then developing is performed for the toner pattern area at a certain bias vbp by a developing apparatus 110 . in this processing , a transfer belt 111 is separated from the surface of the photosensitive body 101 , and the toner pattern is read by the p sensor 112 in a state of being formed on the surface of the photosensitive body . hereinafter , a potential of the pattern area formed at this time is called vsp . furthermore , a potential of the erased area around the toner patterns , in other words , the background area ( hereinafter , called vsg ) is detected by the p sensor 112 . after detecting the potentials vsp and vsg , the toner patterns on the photosensitive body are removed from the surface of the photosensitive body by a cleaning apparatus 113 . thereafter , a normal image forming operation is started through an exposure and charge removing with the charge removing lamp 114 . in other words , the charging voltage vapp is applied to the charging roller 102 to charge the surface of the photosensitive body 101 , an image on the manuscript document is exposed by the exposing apparatus 108 , an electrostatic latent image is formed on the surface of the photosensitive body 101 and developed by the developing apparatus 110 , a toner image ( a picture image ) is transferred to a transfer paper through transfer processing , and the toner image is fixed to the transfer paper by a fixing apparatus to take off the paper outside the machine . in accordance with a density detection value ( potential of toner pattern are a / potential of background area = vsp / vsg ) detected by the p sensor 112 , toner is supplied to the developing apparatus 110 and the operation of the toner supplying roller 118 is controlled . in other words , a toner density in the developing apparatus 110 is controlled by driving rotation of the toner supplying roller 118 for supplying toner from a toner hopper 116 to the developing apparatus 110 and controlling its rotation time . thereby , a density of an image is controlled . according to the third embodiment , the environmental changes or deterioration of sensitivity of the photosensitive body do not cause any potential changes in the toner pattern area for detecting a toner density . therefore , the toner density can be always detected accurately so as to achieve an appropriate toner density control . although a hardware configuration of a fourth embodiment is the same as that of the second embodiment in the above , a part of the image forming control of the cpu 160 is not identical . fig1 shows an outline of a control operation of the cpu 160 in the fourth embodiment . in the fourth embodiment , the cpu 160 executes the above &# 34 ; detecting the charging potential vch &# 34 ; ( step 107a ), &# 34 ; setting applied voltage vap &# 34 ; ( step 108a ), &# 34 ; detecting a pattern density &# 34 ; ( step 122a ), &# 34 ; calculating a toner supplying &# 34 ; ( step 123a ), and &# 34 ; starting toner supplying &# 34 ; ( step 124a ) when the first start input is detected after the power supply is turned on in the same manner as that of the third embodiment ( fig1 ), and thereafter , executes the same processing in the same manner whenever copying by the set count is completed while the power supply is turned on ( steps 107b , 108b , 122b , 123b , and 124b ). in addition , in the fourth embodiment , the cpu checks whether or not the copy count accumulated value written in a register allocated to a nonvolatile memory has reached 1 , 000 ( step 126 ) after a termination of an end cycle . if it has reached 1 , 000 , the cpu applies a charging voltage vapp to the charging roller 102 to adjust a voltage of an exposing lamp , exposes a standard density pattern of a low density on a charged surface of a photosensitive body , and detects a toner density ( vlg for exposed area , vlp for unexposed area ) of the areas developed by a developing apparatus 110 ( exposed area and unexposed area ) of the pattern by using a p sensor 112 ( step 127 ). next , the cpu calculates a voltage vep of the exposing lamp corresponding to a ratio vlp / vlg of a toner density vlp of the unexposed area ( a black written area at a low density ) to a toner density vlg of the exposed area ( a background area ) and writes it in the register allocated to the nonvolatile storage ( step 128 ). then , the copy count accumulated value is cleared ( initialized to 0 ) ( step 129 ). voltage vep is given as a target value to a driver for applying a voltage to the exposing lamp of an exposing apparatus 108 , and the driver performs the constant voltage control for the voltage applied to the exposing lamp so that the voltage of the exposing lamp matches the target value vep . fig1 shows an operation timing of elements related to image forming for a period of setting the voltage vep of the exposing lamp as mentioned above ( steps 127 and 128 ). after the charging voltage vapp is applied to the charging roller 102 to charge the surface of the photosensitive body 101 , a latent image is formed on the photosensitive body 101 and developed at a certain constant bias vb by the developing apparatus 110 by using a background potential detected pattern disposed in the rear side of the forward portion of an optical frame as an original image . at this time , a transfer belt 111 is separated from the surface of the photosensitive body 101 , and a toner pattern of the background potential detected pattern is read by the p sensor 112 with being formed on the surface of the photosensitive body 101 . hereinafter , the potential of the pattern area is called vlp . in addition , the p sensor 112 detects a potential ( vlg ) of the erased area around the toner pattern , in other words , the background area , and calculates a voltage vep of the exposing lamp based on a ratio of the above vlp and vlb values , that is , a density detected value . after detecting the vlp and vlg , the toner pattern on the photosensitive body is removed from the surface of the photosensitive body by a cleaning apparatus 113 . in the fourth embodiment , the exposing lamp voltage vep and the density ratio vlp / vlg are set respectively to the standard values after cleaning an optical system or after replacing the photosensitive body 101 by the other , and thereafter an actual density ratio vlp / vlg is detected at fixed intervals at the above exposing lamp voltage to compensate the lamp voltage based on a ratio of the detected value to the standard value . according to the third embodiment , any environmental changes do not cause any potential changes in the toner pattern area for detecting the intensity of the exposure . therefore , a toner pattern can be always generated accurately so as to achieve an appropriate exposure amount control . in the third and fourth embodiments in the above , although the linear equation vap = a · vch + b , which represents a relationship between the voltage vap applied to the charging roller 102 and the charging potential vch generated on the photosensitive body 101 by the voltage , is determined based on three measurement values , it is also possible to determine the equation based on two or four or more measurement values . if it is determined based on two measurement values , an effect of a measurement error is relatively large . with three measurement values , however , the effect is relatively small . while the effect of the measurement error is decreased by increasing the number of the measurement values , the calculation for determining the linear equation becomes more complicated and time - consuming . as is apparent from the foregoing descriptions of the embodiments according to the present invention , some merits or advantageous functional effects can be found out . in the contact - type charging apparatus of the embodiment according to the present invention in which the temperature is detected and thereby the applied voltage is compensated , the applied voltage needed for making the charging potential equal to a target potential is obtained by detecting the charging potential , and then the compensation rule for the detected temperature is further compensated in accordance with the value ( the value of the applied voltage thus obtained ). in such manner , the charging potential can be stably controlled even though the environmental factors such as temperature , humidity , etc . change . and further , the difference value between the applied voltage needed for making the charging potential equal to the target potential and the other applied voltage determined by the ( detected ) temperature is obtained . the charging is done with the applied voltage obtained by adding the above difference value to the applied voltage determined by the temperature . in such manner , it may be possible to control the charging potential simply and stably . in the contact - type charging apparatus of the other embodiments according to the present invention , the linear equation vap = a · vch + b representing the relationship between the voltage vap applied to the charging roller and the charging potential appearing thereby on the photosensitive body is determined on the basis of the three - points measurement values . if the number of the measurement points is small ( for instance , two points ), the influence exerted on the measurement error becomes large . on the other hand , if the number of the measurement is large ( more than three ), the calculating operation for determining the linear equation becomes complicated and thereby it takes much more times to determine the above linear equation . in the embodiments , the problems as mentioned above can be solved . namely , the influence exerted on the measurement error can be decreased and the calculating operation for determining the linear equation can be made simple ( not complicated ) in order to reduce the time needed for determining the above equation . furthermore , plural rules of the applied voltage compensation are provided for the detected temperature , and an optimum applied voltage compensating rule is selected in accordance with the above - mentioned difference value . in such manner , it may be possible to control the charging potential further stably . obviously , numerous modifications and variations of the present invention are possible in light of the above teachings . it is therefore to be understood that within the scope of the appended claims , the invention may be practiced otherwise than as specifically described herein .