To obtain a stable image by setting gradation correction information in real time in accordance with change of a photosensitive body with time and change of toner electrification quality.

An electric potential of a latent image pattern which is formed on a photosensitive body is detected by a surface electrometer. A gradation correction information changing section 27 changes gradation correction information by a surface electric potential at the time of detection. A gradation correction section 23 corrects a gradation of an image signal by the changed gradation correction information and a correction signal is sent to a gradation pattern generating section 24. The gradation pattern generating section 24 generates an image signal of a gradation pattern by the corrected gradation, which is sent to an exposure control section 3.