An image processing apparatus for forming an image using at least three color materials includes a first generation device for generating a plurality of patches of secondary colors composed of color materials of two different colors; a first measurement device for measuring the patches; a first correction characteristic calculation device for calculating gradation-correction characteristics corresponding to each of the two color materials; a second generation device for generating a group of patches in accordance with an image signal for the two different color materials and an image signal for color materials other than the two color materials by using an image signal corrected on the basis of the gradation-correction characteristics; a second measurement device for measuring patches generated by the second generation device; and a second correction characteristic calculation device for calculating gradation-correction characteristics of image signals for color materials other than the two different color materials.