PURPOSE: To quickly perform potential control so as to prevent large variation of the electrifying capacity of a photosensitive body, by setting a parameter to be used for controlling a latent image forming condition corresponding to the detected temperature of the photosensitive body.

CONSTITUTION: A photosensitive body 1 is at first, uniformly charged with an electrifier 2 and is subjected to destaticization in accordance with a picture by means of picture exposure 3. As a result, a latent image is formed and developed to a visible picture with a developing device 4. The visible toner picture is transferred to a transfer material 6 by means of transferring electrifier 5 and the non-transferred toner is cleaned by a cleaner 7. Thereafter, the photosensitive body 1 is further subjected to uniform destaticization by preexposure 8. Thus one picture forming cycle is completed. By previously measuring the atmospheric temperature T' by using an external temperature sensor, the surface temperature of the photosensitive body 1 measured by a temperature sensor 10 is adjusted with a drum heater 11 by using the external temperature as a reference. Therefore, the environmental stability of the surface potential control of the photosensitive body can be greatly improved and the reliability of control can be increased.