A first color conversion unit converts RGB signals from a line input unit to L*a*b* signals. A lightness range compression unit matches the lightness range of the input color signals with that of an output gamut. If the maximum saturation point on an input line is outside the output gamut, an input line correction unit corrects the point in such a manner that it is mapped onto the surface of the output gamut. If a near-surface determination unit determines that the corrected line is not located on the surface of the output gamut, an out-of-gamut mapping unit maps it onto the surface of the output gamut, whereas if it determines that the corrected line is located near the surface of the output gamut, the surface mapping unit maps all points on the surface of the output gamut. A second color conversion unit converts the mapped L*a*b* signals to CMYK signals.