A document matching process section retrieves a similar image on a basis of the result of a first comparison process for comparing features of a matching key image of first resolution that are stored in a features storage section with features of a matching reference image and the result of a second comparison process for extracting features from a matching key image of second resolution that is stored in an image data storage section and comparing the extracted features with features of the matching reference image that are stored in the features storage section. This allows accurately retrieving a matching reference image similar to the matching key image, even when the matching key image is a zoomed image, an N-up image, or an image of low resolution.