A color conversion technique which places an importance on a color difference and converts an input color outside a gamut into a color inside the gamut to minimize the color difference is available. However, this technique does not determine whether the input color falls inside or outside a visual limit range. Upon processing image data between image input/output devices, image data is input (S201), it is determined whether the input image data falls outside a predetermined gamut (S204), and image data which is determined to fall outside the gamut is converted into a color on the inframarginal portion of or inside the gamut (S206).