When printing image data obtained by an input device, the present invention is intended to effectively utilize the color reproduction area of the printer. An image data processing method of the present invention comprises the steps of scale-down shifting a first color space point outside a maximum color reproduction area of an output unit to a second color space point within the maximum color reproduction area of the output unit; computing an amount of shift by which the first color space point is shifted to the second color space point; converting the second color space point to a third color space point within a color reproduction area smaller than the maximum color reproduction area of the output unit; and outputting an image to the output unit using image data obtained by scale-up shifting the third color space point to a fourth color space point within the maximum color reproduction area of the output unit based on the computed shift amount.