explanation will now be given of exemplary embodiments of the image forming apparatus of the present invention with reference to the drawings . the image forming apparatus 1 according to a first exemplary embodiment of the present invention is shown in fig1 . this image forming apparatus 1 performs image processing based on the color image information sent from an image data input device , such as a personal computer ( not shown ), and forms a color image on recording paper p with an electrophotographic method . the image forming apparatus 1 is provided with image forming units 10 y , 10 m , 10 c , and 10 k that form toner images of the respective colors of yellow ( y ), magenta ( m ), cyan ( c ), and black ( k ). hereinafter , when it is necessary to distinguish between the colors of yellow , magenta , cyan , and black , the letters y , m , c , and k will be added after reference numerals , and when it is not necessary to distinguish between the colors of yellow , magenta , cyan , and black , the letters y , m , c , and k will be omitted . the image forming units 10 y , 10 m , 10 c , and 10 k are arranged in series in the direction of movement of an endless intermediate transfer belt 30 in the sequence of image forming units 10 y , 10 m , 10 c , and 10 k . this intermediate transfer belt 30 is tensioned in a secondary transfer unit around a back up roll 34 , for supporting the intermediate transfer belt 30 from the back surface thereof , and plural tensioning rolls 32 . moreover , the intermediate transfer belt 30 is disposed so as to be inserted between each of photoreceptor drums 12 y , 12 m , 12 c , and 12 k , which serve as the image carriers of the image forming units 10 y , 10 m , 10 c , and 10 k , and their respective primary transfer rolls 16 y , 16 m , 16 c , and 16 k that are disposed opposite thereto . next , the configuration of the image forming units 10 y , 10 m , 10 c , and 10 k and the operation of image formation will be described by way of the image forming unit 10 y that forms the yellow toner image . the surface of the photoreceptor drum 12 y is uniformly charged with the charge roll 13 y . next , a laser beam l corresponding to a yellow image is irradiated to the surface of the photoreceptor drum 12 y from a light exposure apparatus 14 y . thereby , an electrostatic latent image corresponding to the yellow image is formed on the surface of the photoreceptor drum 12 y . the electrostatic latent image corresponding to the yellow image on the surface of the photoreceptor drum 12 y is developed by toner carried on a development roll 18 y , disposed in a development device 15 y , to which a development bias is applied , and forms a yellow toner image . the yellow toner image is primarily transferred onto the intermediate transfer belt 30 by contact - pressure from the primary transfer roll 16 y and electrostatic attraction force due to a transfer bias applied to the primary transfer roll 16 y . in this primary transfer , not all of the yellow toner image is transferred to the intermediate transfer belt 30 ; some remains as transfer residue yellow toner on the photoreceptor drum 12 y . external additives in the toner also adhere to the surface of the photoreceptor drum 12 y . a portion after primary transfer of the photoreceptor drum 12 y passes a position facing a cleaning device 20 y and the transfer residue toner on the surface of the photoreceptor drum 12 y and the like is removed . thereafter , the surface of the photoreceptor drum 12 y is again charged by the charge roll 13 y for the next image formation cycle . as shown in fig1 , in the image forming apparatus 1 , the same image forming process as described above is conducted in the image forming units 10 y , 10 m , 10 c , and 10 k at a timing in consideration of differences in the relative positions of the image forming units 10 y , 10 m , 10 c , and 10 k , the toner images of the respective colors of yellow , magenta , cyan , and black are sequentially superposed on the intermediate transfer belt 30 , and a multiple toner image is formed . then , the multiple toner image is transferred at once from the intermediate transfer belt 30 to the recording paper p , which is conveyed at a predetermined timing to a secondary transfer position a , by electrostatic attraction force of a secondary transfer roll 36 to which a transfer bias is applied . the recording paper p to which the multiple toner image has been transferred is separated from the intermediate transfer belt 30 and thereafter conveyed to a fixing device 31 , where the multiple toner image is fixed to the recording paper p by heat and pressure to form a full - color image . the transfer residue toner on the intermediate transfer belt 30 that has not been transferred to the recording paper p is collected by an intermediate transfer belt cleaner 33 . in such an image forming apparatus 1 , there are process cartridges 62 y , 62 m , 62 c , and 62 k that are each made up integrally from the respective photoreceptor drum 12 disposed in each of the image forming units 10 y , 10 m , 10 c , and 10 k , the charge roll 13 , the cleaning device 20 , and the like . these process cartridges 62 y , 62 m , 62 c , and 62 k are respectively configured so as to be attachable and detachable to / from the image forming device body 60 . as shown in fig2 , the charge roll 13 is disposed in contact with the photoreceptor drum 12 at an upper portion of the photoreceptor drum 12 . the structure of this charge roll 13 is described later . there is a cleaning roll ( for example , a sponge roll ) 50 , which clean the surface of the charge rolls 13 , disposed at the top of the charge roll 13 . the cleaning roll 50 is one in which a sponge layer 54 is formed around the perimeter of a shaft 52 , and the shaft 52 is rotatably supported . the cleaning roll 50 is pressed to the charge roll 13 with a predetermined pressing force by springs ( not shown ) that are arranged at both ends of the shaft 52 . by so doing , the sponge layer 54 of the cleaning roll 50 is elastically deformed along the circumferential surface of the charge roll 13 to form a nip portion . a motor ( not shown ) is connected to a support shaft of the photoreceptor drum 12 , and the photoreceptor drum 12 is driven to rotate with a clockwise rotation as viewed in fig2 ( the direction of the arrow 2 ). moreover , the charge roll 13 is driven by the rotation of the photoreceptor drum 12 and rotates in the direction of the arrow 4 . moreover , the cleaning roll 50 is driven by the rotation of the charge roll 13 and rotates in the direction of the arrow 6 . foreign matter , such as toner and external additives on the surface of the charge roll 13 , are cleaned by the driven rotation of the cleaning roll 50 . it should be noted that , as an alternative configuration , it may be configured such that the charge roll 13 or the cleaning roll 50 has a motor connected thereto and is / are rotated independently . next , explanation will be given of details of the charge roll 13 . the charge roll 13 is disposed in contact with the surface of the photoreceptor drum 12 , and the surface of the photoreceptor drum 12 is charged by applying a direct current voltage , or a direct current voltage on which an alternating voltage is overlapped . the charge roll 13 is provided at the perimeter of a shaft 40 with an electrically resistive resilient layer 42 , as shown in fig3 . the electrically resistive resilient layer 42 is divided , in sequence from the outside , into an electrically resistive layer 46 and a resilient layer 44 which supports the electrically resistive layer 46 . moreover , in order to impart durability and resistance to contamination to the charge roll 13 , a surface layer 48 is formed to the outside of the electrically resistive layer 46 . a material that has electrical conductivity is used for the material of the shaft 40 , and generally iron , copper , brass , stainless steel , aluminum , nickel , and the like are used . moreover , materials other than metals may be used as long as they have conductivity and moderate rigidity , for example , resin moldings with dispersed conductive particles and the like therein , ceramics , and the like , may also be used . moreover , in addition to the profile of a roller , it is also possible use a hollow pipe shape . a material with conductive or semiconductive properties is used for the material of the resilient layer 44 , and generally resin material or rubber material is used in which conductive particles or semiconductive particles are dispersed therein . examples of materials that may be used as resin materials include synthetic resins , such as polyester resin , acrylic resins , melamine resins , epoxy resins , urethane resins , silicone resins , urea resins , polyamide resins , and the like . examples of materials that may be used as rubber materials include ethylene - propylene rubber , polybutadiene , natural rubber , polyisobutylene , chloroprene rubber , silicone rubber , urethane rubber , epichlorohydrin rubber , fluorosilicone rubber , and ethylene oxide rubber , or foamed materials formed by foaming the above . examples of particles that may be used as conductive particles or semiconductive particles include : carbon black ; metals , such as zinc , aluminum , copper , iron , nickel , chromium , and titanium ; metal oxides , such zno — al 2 o 3 , sno 2 — sb 2 o 3 , in 2 o 3 — sno 2 , zno — tio 2 , mgo — al 2 o 3 , feo — tio 2 , tio 2 , sno 2 , sb 2 o 3 , in 2 o 3 , zno , and mgo ; a compounds , such as quaternary ammonium salts . such materials may be used singly , or combinations of two or more thereof may be used . furthermore , the following may be used as required , either used singly or in combinations of two or more thereof : inorganic fillers , such as talc , alumina , and silica ; and organic fillers , such as fine particles of fluororesins or silicone rubbers . conductive particles or semiconductive particles dispersed in a binder resin may be used as materials for the electrically resistive layer 46 and the surface layer 48 , with the electrical resistance thereof being controlled . the resistivity is preferably about 10 3 to about 10 14 ωcm . moreover , the average film thickness of the electrically resistive layer 46 or the surface layer 48 is preferably about 0 . 01 to about 1000 μm . as such a binder resin the following may be used : acrylic resins , cellulose resins , polyamide resins , methoxy methylated nylon , ethoxy methylated nylon , polyurethane resins , polycarbonate resins , polyester resins , polyethylene resins , polyvinyl resins , polyarylate resins , polythiophene resins , polyolefin resins , such as pfa , fep and pet , styrene butadiene resins , melamine resins , epoxy resins , urethane resins , silicone resins , urea resins , and the like . as the conductive particles or semiconductive particles dispersed in the electrically resistive layer 46 and the surface layer 48 , in the same manner as in the resilient layer 44 : the carbon black , metals , metal oxides , and ionic compounds , such as quaternary ammonium salts that exhibit ionic conductivity may be used , either singly or as two or more mixed together . moreover , as required , addition may be made of : antioxidants , such as hindered phenols , and hindered amines ; inorganic fillers , such as clay , kaolin , talc , silica , and alumina ; organic fillers , such as fine particles of fluororesins or silicone resins ; and lubricants , such as silicone oils . these may be added singly or two or more may be added . furthermore , surfactants and charge controlling agents , and the like may be added as required . moreover , the following methods may be used to form these layers : a blade coating method , a wire bar coating method , a spray coating method , an immersion coating method , a bead coating method , an air knife coating method , a curtain coating method , and the like . as shown in fig4 , in an aspect of the present invention , the surface layer 48 of the charge roll 13 having a predetermined surface roughness is formed by kneading the particles 49 into the binder resin 48 a . that is , minute irregularities are formed in the outermost face of the surface layer 48 by the particles 49 . the particles 49 may be either spherical or of undefined shape . here , in order to make the configuration simple to understand , the binder resin 48 a and the particles 49 of the surface layer 48 are shown schematically in fig4 . the ten - point average surface roughness rz of the surface layer 48 is preferably about 3 to about 12 μm , more preferable about 7 to about 12 μm , and particularly preferably about 10 to about 12 μm . by setting the ten - point average surface roughness rz within such ranges , foreign matter , such as a toner and external additives , do not readily adhere to the surface layer 48 , and the resistance of the surface layer 48 to contamination becomes high . on the other hand , when the ten - point average surface roughness rz is less than about 3 μm there is concern that foreign matter , such as a toner and external additives , may adhere thereto . moreover , when the ten - point average surface roughness rz is greater than about 12 μm , toner , paper dust , and the like , readily collect in the irregular parts . furthermore , when the ten - point average surface roughness rz is greater than about 12 μm , local abnormal discharge occurs and uniform charging is hindered by the large height differences of the irregularities , and image defects like fine white deletions tend to occur . here , the ten - point average surface roughness rz in question is the surface roughness specified according to jis b0601 ( 1994 ). although the ten - point average surface roughness rz could be measured using a surface roughness tester or the like , in the present exemplary embodiment , a contact type surface roughness tester ( trade name : surfcom 570a , made by tokyo seimitsu co ., ltd .) is used , in an environment of 23 ° c . and 55 % rh . when measuring the surface layer 48 , the measurement distance is about 2 . 5 mm , and using a diamond tipped stylus ( 5 μm r and 90 ° cone shape ), the location is changed and measurements are carried out three times . the average of these measurements is calculated for the ten - point average surface roughness rz of the surface layer 48 . a material with high electrical resistance is preferable for the material of the particles 49 , and , for example , polymer particles , such as polyimide and methacrylic resins , and inorganic particles , such as silica , or ceramic particles may be used . also , it is preferable that the volume average particle size of the particles 49 is about 1 % to about 50 % of the average film thickness of the surface layer 48 ( the length dimension of the particles 49 is about 1 % to about 50 % relative to the layer thickness dimension of the binder resin 48 a ). it is difficult to obtain the desired ten - point average surface roughness rz if the volume average particle size of the particles 49 is less than about 1 % of the average film thickness of the surface layer 48 . moreover , if the volume average particle size of the particles 49 exceeds about 50 % of the average film thickness of the surface layer 48 then , depending on the blending quantity of the particles 49 , it is difficult to obtain the desired ten - point average surface roughness rz . in the present exemplary embodiment , the average film thickness of the surface layer 48 is for example , about 3 to about 15 μm , and the volume average particle size of the particles 49 is set , for example , at about 1 . 0 to about 7 . 5 μm . here , the blending quantity of the particles 49 is set as about 5 to about 35 volume % relative to the binder resin 48 a . here , the volume average particle size of the particles 49 is the value measured using a coulter counter ( trade name : coulter counter ta - ii , made by coulter co ., ltd .). in this case , measurements are made with the optimal aperture for the particle size level of the particles . also , the average film thickness of the surface layer 48 is the value obtained by cutting off cross sections of the surface layer 48 , carrying out multiple measurements of the cross sections using a scanning electron microscope ( sem ) or a transmission electron microscope ( tem ), and taking the average value therefrom . furthermore , for the material of the particles 49 , semiconductive properties may be imparted the particles by mixing and embedding conductive fine powder into the particles . the conductive fine powders include : metals such as gold , silver , and copper ; carbon black ; and also metal oxides , such as powders of titanium oxide , magnesium oxide , zinc oxide , aluminum oxide , calcium carbonate , aluminum borate , potassium titanate , and calcium titanate ; and fine powders formed by covering the surface of the powder of titanium oxide , zinc oxide , barium sulfate , aluminum borate , or potassium titanate , with the powder of tin oxide , carbon black , or a metal . these may be used independently or combinations of two or more may be used together . the conductive fine powder may be of the same materials as for the above conductive particles included in the binder resin 48 a of the surface layer 48 ( for example , carbon black and the like ). moreover , the material of the particles 49 may be formed from the same resins as for the binder resin 48 a . by using the same resin as used in the binder resin 48 a , good compatibility is achieved and the adhesion becomes high between the particles 49 and binder resin 48 a . preferably used materials for the binder resin 48 a are polyvinylidene fluoride , copolymers of tetrafluoroethylene , polyester , polyimide , and copolymer nylons . examples that may be given of such copolymer nylons are those that include therein one or more of the polymerization units nylon 610 , nylon 11 , and nylon 12 , and other polymerization units which may be included these copolymers are nylon 6 , nylon 66 , and the like . alcohol soluble copolymerization nylon is used in the present exemplary embodiment . on the other hand , as another configuration , as shown in fig5 , depending on the volume average particle size and blending quantity of the particles 49 a , the volume average particle size of the particles 49 a may also be made greater than the average film thickness of the surface layer 48 . in such cases , the desired ten - point average surface roughness rz is controlled by making a state in which a portion of the circumferential surface of the particles 49 a bulges out of the surface of the surface layer 48 . in fig5 , so that the configuration is simple to understand , the binder resin 48 a and the particles 49 a are shown schematically . or , as still another configuration , as shown in fig6 , a thinly coated protective layer 70 may be formed at the outside ( the outermost face ) of the surface layer 48 of the charge roll 13 . the thickness of this protective layer 70 is preferably about 10 μm or less . even if the protective layer 70 is formed , the ten - point average roughness of the outermost surface may be set to the predetermined range by the particles 49 . it should be noted that , so that the configuration thereof may be simply understood , the binder resin 48 a , the particles 49 , and the protective layer 70 are shown schematically in fig6 . next , explanation will be given of the cleaning roll 50 . a free cutting steel , stainless steel , or the like is used as the material of the shaft 52 of the cleaning roll 50 . the material and any surface treatment method of the shaft 52 may be selected according to the application , such as the required sliding properties , and materials that do not have conductivity may be made conductive by carrying out processing using standard treatments , such as plating treatment , and , of course , the material may be used as it is . moreover , in order that the cleaning roll 50 may contact the charge roll 13 through the sponge layer 54 with an appropriate nip pressing force , the shaft 52 material is of a strength that does not bend during nipping , or the shaft diameter is selected to have sufficient rigidity relative to the shaft length . the sponge layer 54 of the cleaning roll 50 includes a foam which has a porous three - dimensional configuration , with cavities and irregular portions ( referred to below as cells ) existing inside or at the surface thereof , and the sponge layer 54 has an elasticity . foaming resin or rubber materials may be used for the sponge layer 54 , and selection may be made from polyurethane , polyethylene , polyamide , olefins , melamine or polypropylene , nbr , epdm , natural rubbers and styrene butadiene rubbers , chloroprene , silicone , and nitrile . thereby , the cleaning roll 50 having many cells may be manufactured cheaply . the cleaning roll 50 efficiently cleans foreign matter , such as external additives while being driven by the rubbing friction of the charge roll 13 , and at the same time it is necessary to ensure that defects are not imparted to the surface of the charge roll 13 due to the cleaning roll 50 rubbing the surface of the charge roll 13 . moreover , it is necessary to ensure that there is no shredding or breakup occurs in the sponge layer 54 over a long period of time . for this reason , polyurethane is especially preferably used as the material of the sponge layer 54 , due to its tearing strength and tensile strength . the polyurethane is not particularly limited . it suffices for there to be an accompaniment of a reaction of a polyol such as polyester polyol , polyether polyester , or acrylic polyol , and an isocyanate such as 2 , 4 - tolylenediisocyanate , 2 , 6 - tolylenediisocyanate , 4 , 4 - diphenylmethane diisocyanate , tolidine diisocyanate , or 1 , 6 - hexamethylene diisocyanate , and it is preferable that a chain extender such as 1 , 4 - butanediol or trimethylolpropane is to be mixed in . further , it is common to cause foaming using water and a foaming agent such as an azo compound like azodicarbonamide or azobisisobutyronitrile . moreover , auxiliaries such as a foaming aid , a foam adjusting agent , and a catalyst may also be added as needed . next , explanation will now be given of the experiments for evaluating the staining properties and the cleaning characteristics of the charge roll 13 . for the charge roll 13 , surface coating is carried out in the state in which the particles 49 of resin are mixed with the surface layer 48 , as shown in fig4 , and samples are prepared by grouping into four grades of respective ten - point average roughnesses rz , of about 1 to about 2 μm , about 3 to about 4 μm , about 7 to about 8 μm , and about 10 to about 12 to μm . evaluation of staining properties and cleaning characteristics is performed using these samples . as methods for evaluating staining properties and cleaning characteristics , in the image forming unit 10 of the image forming apparatus 1 shown in fig1 , a print test is carried out in the state in which the cleaning roll 50 is not attached , and the charge roll 13 is thereby soiled in advance . next , only the photoreceptor drum 12 , the charge roll 13 , and the cleaning roll 50 are installed , and the photoreceptor drum 12 is rotated a predetermined number rotations , and any changes to the charge roll 13 surface are measured . here , in this measuring method , grading is carried out of the changes to the degree of whiteness due to external additives adhering to the surface of the charge roll 13 . g1 is good and g5 is poor , and the grade for the staining properties and cleaning characteristics is worse the higher the figure after g a new charge roll 13 is grade g0 . moreover , the allocation of grades for staining properties and for cleaning characteristics differs . for the grading of cleaning characteristics the level at which there is a satisfactory picture is g3 or below , and the when it exceeds g3 then this indicates an ng level . also , for the grading of staining properties , the worst cases are set to g5 in the 12 evaluation levels ( see table 1 ), and the grades are divided up to g1 so as to be roughly equal . alcohol soluble copolymerization nylons are used as the binder resin 48 a of the surface layer 48 , carbon black as a conductive fine powder is embedded in the particles 49 , and the samples are prepared by changing the particle size of the particles 49 and blending quantities to prepare sample materials . there are three levels for the average film thickness of the surface layer 48 , about 3 to about 4 μm , about 8 to about 9 μm , and about 14 to about 15 μm . when the average film thickness of the surface layer 48 is about 3 to about 4 μm , particles whose volume average particle sizes are about 1 . 5 μm are used , and when the average film thickness of the surface layer 48 is about 8 to about 9 μm , particles whose volume average particle sizes are about 4 μm are used , and when the average film thickness about 14 to about 15 μm , particles whose volume average particle sizes are about 8 μm are used . the blending quantity of the particles is determined according to the roughness , and is about 15 % to about 70 % ( as a weight % relative to the surface layer resin solid content ). the evaluation results for the staining properties and cleaning characteristics are shown in table 1 and table 2 . it may be seen from the results shown in tables 1 and 2 that staining properties and cleaning characteristics may improve as the ten - point average surface roughness rz increases . furthermore , a standard print test is carried out using the image forming apparatus 1 shown in fig1 using the same charge roll 13 , and experiments are conducted to examine the correlation between the numeric value of cleaning characteristics and actual defects ( image defects ). from these experiments it may be shown that for cleaning characteristics of g3 or below in table 2 , good printing may be maintained for 100 , 000 sheets . from the results of these experiments , it may be seen that the ten - point average surface roughness rz is preferable 3 μm or more . furthermore , for a ten - point average surface roughness rz of 3 μm or more , there is no variation generated in the cleaning characteristics by the location along the shaft direction of the charge roll 13 , foreign matter may be removed almost uniformly , and also the amount of foreign matter adhering itself may be reduced . furthermore , with regard to the upper limit of the ten - point average surface roughness rz , the cleaning characteristics in the region close to 10 μm or more may maintain good values . however , with respect to the initial charging uniformity , if the ten - point average surface roughness rz exceeds 12 μm , then uneven charge may occur , with white spots and the like , and initial image quality may not be achievable . from the above , it may be effective to mix the particles 49 with the surface layer 48 , as described above , so that the ten - point average surface roughness rz becomes 3 μm to 12 μm . next , sandblasting is used as the method for making coarse the roughness of the surface layer of the charge roll 13 , and a sample ( comparative example ) is prepared so that the same ten - point average surface roughness rz as in the example is achieved . evaluation of staining properties and cleaning characteristics is performed using this sample ( comparative example ). it may be possible to achieve the desired ten - point average surface roughness rz even if sandblasting is used as the method , however , as shown in table 3 , the resistance value that may be important for a charge roll rises to the extent of one figure under all conditions . this may show inferior electrostatic properties , especially in low temperature low humidity environments . moreover , as shown in table 3 besides the rise of resistance , the increase in the variation ( sigma ) in resistance may also be important , and generally when this exceeds 0 . 1 , it may be difficult to achieve uniform charging . also it may be shown that control of the ten - point average surface roughness rz by the particles 49 of the aspect of the present invention may have a large effect , even in comparison with the results by the sandblasting . if the volume average particle size of the particles 49 is too small then roughness may not be readily obtained , even if the blending quantity is increased . moreover , if the volume average particle size of the particles 49 is too large , then the particles 49 tend to readily fall out . about 1 % to about 50 % of the average film thickness of the surface layer 48 may be suitable for the volume average particle size of the particles 49 . moreover , by mixing and embedding carbon black as the conductive fine powder into the particles 49 , the variation in the surface resistance of the charge roll 13 may be suppressed , and abnormal discharge does not readily occur . it should be noted that , although the image forming apparatus 1 of the above exemplary embodiment is a configuration with the image forming units of yellow , magenta , cyan , and black disposed in series along the conveying direction of the intermediate transfer belt , the present invention is not limited to this structure . the present invention may also be applied , for example , to a configuration in which toner images of each color are formed one by one on a photoreceptor drum using a rotary development apparatus . the foregoing description of the exemplary embodiment has been provided for the purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise forms disclosed . obviously , many modifications and variations will be apparent to practitioners skilled in the art . the exemplary embodiment are chosen and described in order to best explain the principles of the invention and its practical applications , thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated . it is intended that the scope of the invention be defined by the following claims and their equivalents .