PURPOSE: To prevent black solid on a nonimage part and density unevenness in the longitudinal direction, by detecting an original size at the same time as start of copying, and erasing charge on the part excluding the area corresponding to the original.

CONSTITUTION: When an original different in size from a copying sheet is used or an original is placed on a position different from a normal set position and a switch is operated, a halogen lamp 2 is turned on, and the original 1 is illuminated. The light reflected passes through a lens 3 and an opaque mirror 4, and falls on a photodiode array. Its output passes through a clock pulse generator 10, a timing circuit 8, a shift register 7, and an interface 9, and enters a microprocessor 11. A signal corresponding tothe part except the original 1 and not reflecting light is fed via a DA converter 12 and a switch control circuit 13 to an erasing lamp 14 to erase the charge on said part of the photoreceptor, thus permitting black solid on the nonimage part and uneven density in the longitudinal direction to be prevented.