PURPOSE: To exactly erase an unnecessary image, by installing an unnecessary image erasing lamp at a position away from a developing device, and also flickering it.

CONSTITUTION: An unnecessary image erasing lamp 10 is provided between an exposing point 3 and a charger 12, and the following control is executed. That is to say, registration of a form 9 is executed by the fall of a signal (a) which is generated when the front end of a light shielding plate 7 provided on a carriage 5 passes through a photodetector 8. Also, the length of the light shielding plate 7 is made equal in distance to the unnecessary image erasing lamp 10 from the exposing point 3, so that frickering control of the lamp 10 is executed by a signal (b) which is generated when the rear end of the light shielding plate 7 passes through the photodetector 8. According to such constitution, this device is scarcely stained by a toner leaked from a developing device, by which an unnecessary image can be erased exactly.