Color data for a color image device having a first color range (gamut) are converted into color data for a color image device having a second color range (gamut). For preventing colors from being altered when color data are converted, first color data in the first color range (gamut) of the first image device is converted into corresponding colors by referring to a classification table representing the correspondence between colors determined based on human color perception and the first color data, and then the first color data is converted into second color data in the second color range (gamut) of the second image device with respect to each of the classified colors. Since each of the colors based on the human color perception is converted, colors are prevented from being altered due to the conversion. Because each of the colors of a color image is converted, the clearness of the color image will not be lost.