PURPOSE: To prevent the sticking of a carrier to the nonimage part of a photosensitive body by executing writing on all regions by means of optical writing. and making the potential by electrification of the photosensitive body low, when the value of output applied to a grid is larger than a rated value, regardless of the value of the output applied to the charger wire of an electrifier.

CONSTITUTION: An electrifying charger power source 10 and a grid power source 11 are connected to the charger wire 2b and grid 2c of a sutocorotron electrifier 2, respectively, to apply a voltage and a current. On the other hand, a developing bias power source 12 is connected to a developing roller 4a, to impart a bias voltage and generate a developing potential. When the value of the output applied to the charger wire 2b of the electrifier 2, is larger than the rated value by a prescribed value or more, regardless of the value of the output applied to the charger wire 2b of the electrifier 2, the writing is attained on all regions capable of writing, with a laser writing beam 3, in disregard of original writing data, and the potential by electrification of the photosensitive body 1 can be made low.