An image forming apparatus using a plurality of colorants for image formation, includes a generation unit which generates bitmap data, in which each pixel has a gradation value of each color of the colorants, with respect to an object of the image formation; a classification unit which classifies the pixels into a plurality of predetermined groups based on the bitmap data and generates classification data corresponding to the classified groups for each pixel; a detection unit which detects, based on the classification data, a range, in which a correction processing of modifying the gradation values has to be executed, on the bitmap data; and a correction unit which executes the correction processing to the detected range, wherein the image formation is executed based on bitmap data after the correction processing.