A gradation-correcting curve to correct a gradation of an input image is generated. Gradation correction based on the gradation-correcting curve is made to a boundary pixel of a color-reproduction space at the same saturation as that of a target lattice point and at a hue of the target lattice point. An equal-saturation line is set by using the boundary pixel of the color-reproduction space after the gradation correction. A saturation-correction amount of the target lattice point is decided based on the gradation-correcting curve and the equal-saturation line.