A representative point for pixel value adjustment at which the same pixel value is to be set is selected from each of a correction image to be corrected and a reference image of a mosaic image. A tone curve that defines an output pixel value with respect to an input pixel value is generated by a line that passes through a pixel-value-conversion coordinate point defined by pixel values indicating the representative points selected from the correction image and the reference image, which are set as input and output values. This line is generated by a spline curve. The pixel value of the correction image is converted based on the generated tone curve.