PURPOSE: To evade excessive compensation and to prevent a white part from turning gray by performing compensation for improper image formation conditions due to the potential drop of the surface of a photosensitive body according to the ambient temperature of the photosensitive body when outside air temperature is high, and controlling image formation conditions according to the outside air temperature when the outside temperature is low.

CONSTITUTION: When the outside air temperature is high, the cooling effect of the photosensitive body 1 by a form is small, so the temperature of the photosensitive body 1 is high. In this case, temperature compensation is performed based upon the ambient temperature of the photosensitive body measured by a 2nd means 4, so deterioration of image formation conditions due to a potential drop can be compensated sufficiently. When the outside air temperature is low, on the other hand, the cooling effect of the photosensitive body 1 by the form increases and the temperature of the photosensitive body 1 is held low, so the difference from the measured temperature of a 2nd measuring means 4 increases. In this case, temperature compensation for the image formation conditions is performed with the output of a 1st measuring means 35. Consequently, no excessive compensation is performed and any white part does not turn gray.