Each of a plurality of input images is smoothed. A plurality of smoothed images different from each other in the degree of smoothing thus results. Each of first and second synthesizers calculate an edge intensity based on an intermediate synthesized image generated from at least one smoothed image and having a relatively large degree of smoothing and another smoothed image having a relatively small degree of smoothing, and synthesizes the intermediate synthesized image and the smoothed image based on the edge intensity, thereby generating a synthesized smoothed image. A second synthesized smoothed image generated by the second synthesizer is output as a final synthesized smoothed image representing an illumination component, and is then used as a coefficient for converting a pixel value.