The blocking unit generates first block property data having a first value if all pixels in the block image do not have a color of the color plane, and generates first block property data having a second value if at least one pixel in the block image has the color. The gamma correction unit performs gamma correction for the block image having the second value and does not perform gamma correction for the block image having the first value. The screen process unit generates a block dot image by performing a screen process for the block image after the gamma correction. The toner adhesion amount calculating unit identifies values of the first block property data of block images adjacent to an objective block image, and identifies the toner adhesion amount of the object block as a value changed correspondingly to the identified values of the first block property data.