Variations of image density and gradation reproducibility upon image formation include short-term variations resulting from variations of apparatus environments, and long-term variations resulting from aging of the photosensitive bodies and toners. In order to standardize the density and gradation reproducibility of a copy image and print image, these variations must be corrected together. For this purpose, test print 1 is formed (S101), a correction coefficient ka of the contrast potential for image formation is optimized based on density information obtained in step S102 (S103), and the grid potential and developing bias potential are set to obtain the contrast potential (S104). Test print 2 is formed (S105), the relationship between the laser output and density is calculated from density information obtained in step S106 (S107), and gamma conversion characteristics are set (S108).