PURPOSE: To stabilize the density of a print picture by controlling the quantity of electrification to a photosensitive body so that the measured surface potential of the photosensitive body obtains a prescribed value and coping with the variation of a condition for image formation.

CONSTITUTION: A means 4 measures the surface potential of a photosensitive body when the optical image of a light emitting device array 14 is irradiated to the photosensitive body 1 to form the latent image of an origianl picture. A means 24 controls the quantity of electrification to the photosensitive body 1 so that the surface potential of the photosensitive body 1 obtains a prescribed value. Consequently, the ununifomrity of sensitivity characteristics of the photosensitive body 1 is compensated to cope with the variation of the condition for the image formation, and thus, a print picture having stable density is obtained.