A color gamut of image data is appropriately subjected to a smoothing processing to print an image of favorable color reproduction and gradation reproduction. Specifically, when an average lightness is equal to or lower than a blacking lightness, the object pixel is not subjected to the smoothing processing. This can conserve the signal value of the object pixel to prevent a pixel originally having a signal value higher than the blacking lightness from being converted by the gamut compression to a black point. In this manner, a pixel for which the object pixel has the average lightness equal to or lower than the blacking lightness is blacked and a pixel having a signal value higher than the blacking lightness is not compressed to the black point. The resultant image has no deteriorated gradation characteristic when being macroscopically observed.