PURPOSE: To maintain gradation characteristics excellently by detecting the latent image electric potential of a reference gradation pattern formed on the photosensitive body of an electrophotographic device, and controlling the latent image electric potential to be constant at all times.

CONSTITUTION: The apparatus is equipped with converting means for converting input image data 14 in accordance with converting information, RAM11 for storing electrostatic latent image electric potential data with respect to the input coverage value in its initial state, and an optical system 3 for exposing the reference pattern of the input coverage pattern on the photosensitive body and forming an electric latent image. The apparatus is also equipped with an electrostatic potential sensor 4 for detecting the electric potential of the electrostatic latent image in the reference pattern formed by the optical system 3, and means for converting the converted information of image data converting means such that, in comparing detected electric potential data and the electric potential data stored in RAM 11, and, if the difference of both electric potential data is larger than the set value, the electrostatic latent image potential to the identical input image data value conforms to the electrostatic latent image potential in its initial state.