A saturation ratio calculation unit calculates a saturation ratio of the saturation of an outer circumference of a color gamut of an input image to the saturation of an outer circumference of a color gamut of an output image. A histogram generation unit generates a histogram in which a plurality of pixels selected as pixels outside the color gamut are counted for each pair of lightness and hue. A magnification determination unit determines, within a range of not less than 1 and not more than the saturation ratio, a magnification by which the chromaticity of the input image to be subjected to the color gamut conversion is to be multiplied for each pair of lightness and hue based on the histogram.