In a process of generating color separation data, if printing is performed with use of a particular color printing material in addition to basic printing material, the process can effectively use a gamut area which can be enlarged with use of the particular color printing material. Specifically, the process calculates a granularity for each equal-lightness line. For each of such equal-lightness lines, color separation data is obtained which relates to the mixture ratio of the yellow ink to the light magenta ink corresponding to the best granularity. Then, data on lattice points corresponding to the equal-lightness line is changed with the obtained color separation data on the mixture ratio of the yellow ink to the light magenta ink which corresponds to the best granularity. Such obtained color separation data allows the yellow and light magenta inks and the particular color ink red to be used in a low saturation part.