A combination to minimize moiré can be determined theoretically or empirically in four-color printing, but it is very difficult to find a combination to minimize moiré in printing using five or more colors. To solve this problem, image data is color-separated into image data corresponding to plural colorant. Multilevel dither processing is performed to the image data corresponding to the plural colorant. At this time, dither matrices having the same screen angle, the same screen ruling, and different threshold setting methods are applied to the respective image data corresponding to dark colorant and light colorant having the same hue and different lightness values.