referring now to the drawings , wherein like reference numerals designate identical or corresponding parts throughout the several views , and in particular to fig1 , one example is provided of an image - forming unit 20 to form a full - color image , according to the present invention . in fig1 , a control unit 21 is provided to control the image - forming unit 20 to operate . a writing unit 1 is arranged in the image - forming unit 20 to receive an input of image data f 3 transmitted from either an image reading section or a host computer . a photo - sensitive drum ( pc drum ) 3 m is also arranged below the writing unit 1 to form a latent image of magenta in accordance with color separation data of magenta included in the image data f 3 . similarly , photo - sensitive drums 3 c , 3 y and 3 bk are arranged below the writing unit 1 to form latent images of cyan , yellow , and black in accordance with cyan , yellow , and black color separation data included in the image data f 3 , respectively . a plurality of developing devices 2 m , 2 c , 2 y , and 2 bk are arranged in the vicinity of the respective pc drums 3 m , 3 c , 3 y , and 3 bk to develop the latent images formed thereon to be visible images of m , c , y , and bk . further , a transfer belt 7 is arranged in the image forming unit 20 in contact with the respective pc drums 3 m , 3 c , 3 y , and 3 bk and is rotated by a pair of driven rollers 6 a and 6 b and a guiding roller 11 . on the opposite side to the respective pc drums 3 m , 3 c , 3 y , and 3 bk of the transfer belt 7 , a plurality of transfer devices 5 m , 5 c , 5 y , and 5 bk are arranged to transfer respective m , c , y , and bk visible images on the pc drums 3 m , 3 c , 3 y , and 3 bk to the transfer belt 7 one after another . a detection sensor unit 12 is arranged to detect a seam mark and full - color - matching patterns formed on the transfer belt 7 . as illustrated in fig2 , the detection sensor unit 12 includes a pair of pattern detection sensors 15 a and 15 b separately arranged apart from each other in the vicinity of the transfer belt 7 on an arrangement line d extending perpendicular to a conveyance direction of the transfer belt 7 . also included is a seam mark detection sensor 16 arranged on the arrangement line d opposing a region excluding the color - matching patterns on the transfer belt 7 . referring again to fig1 , a cleaning unit 10 is arranged in the vicinity of the transfer belt 7 between the guide roller 11 and driven roller 6 a to clean the transfer belt 7 after completion of an image forming operation . further , the control unit 21 is connected to an output terminal ( not shown ) of the detection sensor unit 12 . a control calculation unit 13 is also connected to the detection sensor unit 12 to calculate detection signals detected by the detection sensor unit 12 and to output an adjustment signal to control an image forming operation to be adjusted . a general control unit 14 is connected to the control calculation unit 13 to generally control the image forming operation . a display 22 is connected to the general control unit 14 to display various information . an image formation control signal f 2 is input to the image - forming unit 20 from the general control unit 14 . further , the control calculation unit 13 includes a replacement number setting device , which sets a number of seam mark detection signals output from the seam mark detection sensor 16 to determine a replacement time for a new transfer belt 7 . a replacement notification - selecting device is also provided to selectively determine if an advance notice of replacement is to be given when a prescribed number of the seam marks is detected , and accordingly , that of the detection signals is counted . an exemplary operation of the above - mentioned system is now described . the image - forming unit 20 executes an image forming operation in accordance with an image formation control signal f 2 input from the general control unit 14 to the image - forming unit 20 . specifically , the pair of driven rollers 6 a and 6 b rotates the transfer belt 7 at a prescribed surface speed in a direction shown by an arrow . when the pc drums 3 m , 3 c , 3 y , and 3 bk are rotated clockwise at a same surface speed as that of the transfer belt 7 , the writing unit 1 initially extracts and obtains m image data from the image data f 3 . an optical unit serving as the writing unit 1 formed from a laser , a polygon mirror , f - theta lens , and a reflection mirror writes an image in accordance with the m image data , and thereby a magenta latent image ( herein after referred to as an m - latent image ) is formed on the pc drum 3 m . when the m - latent image passes through the developing unit 2 m while the pc drum 3 m is rotated , the m - latent image formed on the pc drum 3 m is developed by the developing unit 2 m with m toner , and thereby a m - visible image is formed thereon . further , when the m visible image then passes through a transfer roller 2 m while the pc drum 3 m rotates , the m - visible image is transferred to the transfer belt 7 under influence of a bias voltage impressed onto the transfer roller 5 m . in accordance with the c - image data extracted and obtained from the image data f 3 , the optical unit 1 forms a cyan latent image ( herein after referred to as a c - latent image ) on the pc drum 3 c with a delay of a prescribed time period from the optical writing for the m - latent image on the pc drum 3 m . subsequently , the c - latent image formed on the pc drum 3 c is developed by the developing unit 2 c with the c - toner ; thereby a c - visible image is formed on the pc drum 3 c . in synchronism with the leading end of the c - visible image arriving at a position of the transfer roller 5 c , the leading end of the m - visible image on the transfer belt 7 arrives at the transfer position . then , the c - visible image on the pc drum 3 c is sequentially transferred by the transfer roller 5 c to overly on the m - visible image on the transfer belt 7 , and thereby the m and c - visible images are sequentially formed on the transfer belt 7 . similarly , the optical unit 1 forms a yellow latent image ( herein after referred to as a y - latent image ) on the pc drum 3 y with a delay of a prescribed time . the y - latent image is then developed by the developing device 2 y with y toner . the y visual image is then transferred by the transfer roller 5 y to overly the m and c , visual images conveyed to a transfer position of the transfer roller 5 y . thereby , the m , c , and y visual images are sequentially formed and superposed on the transfer belt 7 . then , the optical unit forms a black latent image as a bk - image ( herein after referred to as a bk - latent image ) on the pc drum 3 bk with a delay of a prescribed time period . the bk - latent image is then developed by the developing device 2 bk with bk toner . the bk visual image is then transferred by the transfer roller 5 bk to overly the m , c , and bk visual images conveyed to a position of the transfer roller 5 bk . thereby , the m , c , y , and bk visual images are sequentially formed and superposed on the transfer belt 7 . thus , a full - color image of the m , c , y , and bk visual images is formed on the transfer belt 7 in accordance with the image data f 3 . the full - color image is then transferred at once to a transfer sheet ( not shown ) in the vicinity of the guiding roller 11 from the transfer belt 7 . the full - color image then receives a fixing process in accordance with the image data , and thereby the full - color image forming process is completed . beside the full - color image forming process , a plurality of full - color matching patterns 18 a and 18 b each having m , c , y , and bk visual images for color offset adjustment use are periodically or optionally formed on the pc member in accordance with prescribed instructions . the full - color matching patterns 18 a and 18 b are then transferred to both end sides of the transfer belt 7 . a seam mark 17 is unavoidably formed at a seam of the transfer belt 7 to be detected by the seam detection sensor 16 . as illustrated in fig2 , the pattern detection sensors 15 a and 15 b are separately arranged from each other in the vicinity of the transfer belt 7 on the arrangement line d drawn perpendicular to the conveyance direction of the transfer belt 7 . specifically , the pattern detection sensors 15 a and 15 b are positioned downstream of the transfer positions for the full - color matching patterns 18 a and 18 b on the transfer belt 7 . these pattern detection sensors 15 a and 15 b detect the full - color matching patterns 18 a and 18 b , respectively , as illustrated in fig5 . the seam mark detection sensor 16 is also arranged on the arrangement line d opposing a region of the transfer belt 7 other than where the color - matching patterns 18 a , 18 b are formed . detection signals continuously output from the pattern detection sensors 15 a and 15 b are sampled at a prescribed frequency and input to the control calculation unit 13 and receive aid conversion into digital signals ( i . e ., voltage values ) one after another . the digital signals are sequentially stored in a memory ( not shown ) provided in the control calculation unit 13 . specifically , the control calculation unit 13 may execute an a / d conversion process by converting the detection signal as an analog signal into a digital signal using , e . g ., a conventional saw tooth state wave signal and reference clock , which reference clock determines a frequency of sampling . the control calculation unit 13 may then count a number of reference clocks ( i . e ., sampling clocks ) until the detection signal arrives at the reference signal level as illustrated in fig5 as a first count value . the reference signal level is set in proportion to a normal density of a pattern , for example . a number of reference clocks is subsequently calculated by the control calculation unit 13 after the detection signal arrives and decreases from the reference signal level until the detection signal arrives again at the reference signal level as a second count value . the control calculation unit 13 may add half a difference between the first and second count values to the first count value to obtain a center of the first and second count values to determine a central position within a mark . the control calculation unit 13 may then store the thus counted values in the memory as a center positional information of the detected mark in the pattern one after another . the seam mark detection signal output from the seam mark detection sensor 16 also receives similar a / d conversion , clock number counting , and positional information calculation , and storage as performed for the pattern detection signals from the control calculation unit 13 . then , the control calculation unit 13 may generate a series of count values na1 to nan in correspondence with the bk , y , c , and m pattern mark positions detected by the pattern detection sensor 15 a . the control calculation unit 13 may also generate a count value nc in correspondence with the seam mark position detected by the seam mark detection sensor 16 . then , the below described calculation is performed wherein legend “ a ” represents a prescribed approximation judgment value : similarly , a series of count values nb 1 to nbn are generated in correspondence with the bk , y , c , and m pattern mark positions detected by the pattern detection sensor 15 b , and the below described calculation is similarly performed : then , where nax and nbx simultaneously meet these formulas ( 1 ) and ( 2 ) are determined and deleted from the memory by regarding that the seam mark is improperly detected by the pattern detection sensors 15 a and 15 b as a part of the pattern mark as illustrated in fig6 . specifically , these approximate control values are highly provably obtained from the seam 17 making a right angle with the traveling direction of the transfer belt 7 . as a result , only count values corresponding to the actually formed patterns 18 a , 18 b can be stored in the memory while excluding the count value corresponding to the seam mark 17 detected by the pattern detection sensors 15 a and 15 b . then , the control calculation unit 13 calculates an adjustment signal f 1 adjusting and controlling the image forming apparatus to operate in accordance with the count values corresponding to full - color matching patterns while excluding erroneous detection signals . the adjustment signal f 1 is then output from the control calculation unit 13 to the general control unit 14 . the general control unit 14 generates an image formation control signal f 2 controlling the image - forming unit 20 to form an image in accordance with the adjustment signal f 1 . the image formation control signal f 2 is input to the image forming unit 20 . the image - forming unit 20 performs an image - forming operation in accordance with the image formation control signal f 2 . thus , the image forming unit 20 precisely adjusts an amount of toner supplied to developing units 2 m , 2 c , 2 y , and 2 bk , bias voltages impressed to the transfer rollers 5 m , 5 c , 5 y , and 5 bk , color offset , or the like , in accordance with the detection signals of the full - color matching patterns . as a result , a high quality image is always formed in accordance with the image data f 3 . an operation of an advance notice mode notifying the necessity of replacement of a currently used transfer belt is now described with reference to fig4 . as shown in fig4 , the count value nc corresponding to the seam mark detection signals generated by the seam mark detection sensor 16 is stored in the memory , as mentioned earlier , in step s 1 . if it is determined that the transfer belt 7 includes a seam 17 , the process goes to step s 2 . then , it is determined if the count value nc exceeds a prescribed setting value . if the prescribed setting value is determined as positive , yes in step s 2 , the process goes to step s 3 . it is then determined if the advance notice mode is set . if the determination is determined as yes in step s 3 , the process goes to step s 4 . messages indicating the need to replace the current transfer belt 7 are then displayed on a display 22 by the general control unit 14 in response to an instruction from the control calculation unit 13 . thus , when a user selectively sets the advance notice mode to the control calculation unit 13 , a usage life of the transfer belt 7 is checked in accordance with a number of detections of the seam mark 17 and the replacement , i . e . the need to replace the current transfer belt 7 , status is displayed at a prescribed time as mentioned above . as a result , a high quality image is continuously formed while appropriately replacing a used transfer belt 7 with a new one in accordance with an operational condition of the image forming apparatus . in contrast , if the user does not select the advance notice mode , the transfer belt 7 is periodically replaced at a prescribed interval . mechanisms and processes set forth in the present invention may be implemented using one or more conventional general - purpose microprocessors and / or signal processors programmed according to the teachings in the present specification as will be appreciated by those skilled in the relevant arts . appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure , as will also be apparent to those skilled in the relevant arts . however , as will be readily apparent to those skilled in the art , the present invention also may be implemented by the preparation of application - specific integrated circuits by interconnecting an appropriate network of conventional component circuits or by a combination thereof with one or more conventional general purpose microprocessors and / or signal processors programmed accordingly . the present invention thus also includes a computer - based product which may be hosted on a storage medium and include , but is not limited to , any type of disk including floppy disks , optical disks , cd - roms , magnet - optical disks , roms , rams , eproms , eeproms , flash memory , magnetic or optical cards , or any type of media suitable for storing electronic instructions . 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 present invention may be practiced otherwise than as specifically described herein .