An object of the present invention is to extract an image closest to a reference image from among a plurality of candidate images. A plurality of candidate images similar to the reference image are extracted from a plurality of images by utilizing granulometry. The plurality of candidate images is transformed based on the reference image. Mutual information shared by each of the transformed candidate images and the reference image is calculated. A candidate image sharing the largest amount of mutual information with the reference image is selected from among the plurality of candidate images.