1. An image processing apparatus, including: a features extraction section for extracting, from image data, features of an image that are included in the image data; features storage means for storing features of a reference image and identification information of the reference image in such a manner that the features are related to the identification information; and a similar image retrieval section for comparing features of an image designated as a matching key image with features of at least one matching reference image which is an image to be matched out of reference images whose features are stored in the features storage means, so as to calculate a similarity between the matching key image and each of said at least one matching reference image, and retrieving a similar image similar to the matching key image on a basis of the calculated similarity; the image processing apparatus comprising a resolution conversion section for converting resolution of the matching key image into first resolution and second resolution different from the first resolution, the similar image retrieval section retrieving the similar image on a basis of (i) a result of a first comparison process in which the features extraction section compares features extracted by the features extraction section from the matching key image of the first resolution with features of each of said at least one matching reference image that are stored in the features storage means and (ii) a result of a second comparison process in which the features extraction section compares features extracted by the features extraction section from the matching key image of the second resolution with features of each of said at least one matching reference image that are stored in the features storage means.