1. An image processing apparatus comprising: a features storage device; an image data storage device; a resolution conversion section for converting resolution of image data into a first resolution and a second resolution that is lower than the first resolution; a storage process section for causing the features storage device and the image data storage device to store a reference image in such a manner that (a) the features storage device is caused to store (i) features, which have been extracted by the features extraction section out of image data of the reference image, which image data has been converted to the first resolution by the resolution conversion section, and (ii) identification information of the reference image in such a manner that the features are related to the identification information, and (b) the image data storage device is caused to store (i) image data of the reference image, which image data has been converted to the second resolution by the resolution conversion section, and (ii) the identification information of the reference image in such a manner that the image data is related to the identification information; and a similar image retrieval section for retrieving a similar image similar to a matching key image out 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 device, the resolution conversion section being configured to convert resolution of the matching key image into the first resolution and the second resolution, the features extraction section being configured to extract (i) features of the matching key image which has been converted to the first resolution, (ii) features of the matching key image which has been converted to the second resolution, and (iii) features of each of said at least one matching reference image of the second resolution stored in the image data storage device, the similar image retrieval section being configured to carry out: (1) a first similarity calculation process for calculating a first similarity between the matching key image and each of said at least one matching reference image of the first resolution by comparing (i) features, which have been extracted by the features extraction section, of the matching key image of the first resolution with (ii) features of each of said at least one matching reference image of the first resolution stored in the features storage device, and (2) a second similarity calculation process for calculating a second similarity between the matching key image and each of said at least one matching reference image of the second resolution by comparing (i) features, which have been extracted by the features extraction section, of the matching key image of the second resolution with (ii) features, which have been extracted by the features extraction section, of each of said at least one matching reference image of the second resolution, and the similar image retrieval section being configured to further carry out: (a) a retrieval process for retrieving, as the similar image, (i) a matching reference image having an added similarity highest among at least one added similarity each of which is defined by the first similarity and the second similarity added to each of said at least one matching reference image, (ii) at least one matching reference image having an added similarity which is equal to or higher than a predetermined similarity set in advance, (iii) a matching reference image having a maximum similarity selected from the group consisting of the similarity calculated in the first similarity calculation process and the similarity calculated in the second similarity calculation process, or (iv) a matching reference image having the maximum similarity which is equal to or higher than a predetermined similarity, or (b) one of the first similarity calculation process and the second similarity calculation process, and then (i) the similar image retrieval section carrying out a first similar image retrieval process for retrieving, as the similar image, a matching reference image having a similarity, which has been calculated in said one of the first similarity calculation process and the second similarity calculation process, is equal to or higher than a predetermined similarity, (ii) in a case where the matching reference image, which has the similarity equal to or higher than the predetermined similarity, is not retrieved in the first similar image retrieval process, the similar image retrieval section carrying out the other of the first similarity calculation process and the second similarity calculation process, and (iii) the similar image retrieval section carrying out a second similar image retrieval process for retrieving, as the similar image, a matching reference image having a similarity, which has been calculated in said other of the first similarity calculation process and the second similarity calculation process, is equal to or higher than a predetermined similarity, and the features extraction section, the resolution conversion section, the storage process section, and the similar image retrieval section being caused to function by a processor executing programs for carrying out the process(es) of each of the sections.