A gradation image forming apparatus comprises a gradation data generation section. The gradation data generation section generates gradation data in each pixel of a gradation image based on at least one table of a reference color number corresponding to a pixel value of a reference color which is a start color of the gradation image, at least one table of a difference number corresponding to a difference in pixel values between adjacent pixels in the gradation image, and a pixel value of each pixel of the gradation image.