PURPOSE: To detect an uncommonness of a photoreceptor surface automatically and mechanically by comparing the level of reflected light with a comparison value proportional to a minimum value in an unexposed photoreceptor surface, or a comparison value proportional to a maximum value in an exposed photoreceptor surface.

CONSTITUTION: While an exposed photoreceptor surface moves right under a phototransfer 302, a maximum level and a minimum level are held in a maximum and minimum holding circuits 322 and 321, and their difference level appears at the output of a differential amplifier 40. Those held levels are compared with prescribed comparison values by comparators 44 and 45, respectively. Therefore, if there is a black stripe on the photoreceptor surface, the comparators 44 and 45 both generate high-level signals and a discrimination output (j) has a high level. If there is a white part, the comparator 44 outputs the high- level signal and the comparator 45 outputs a low-level signal to obtain a high- level discrimination output (h). When the photoreceptor surface is normal, the comparator 44 has the high-level output.