1. An image search apparatus for searching for a search-target image corresponding to an input image from among a plurality of search-target images, comprising: a characteristic partial image detection unit for detecting a characteristic partial image of each search-target image based on a dissimilarity level of a partial image at a corresponding position among a plurality of search-target images, the characteristic partial image detection unit comprising: a candidate extraction unit for extracting a candidate of a characteristic partial image from each search-target image; an appearance number counting unit for counting, for each candidate of a characteristic partial image of an arbitrary search-target image extracted by the candidate extraction unit, the number of times that the partial image of the candidate appears in other search-target images; and a detection unit for detecting a candidate with a low appearance count counted by the appearance number counting unit as a characteristic partial image; and a search unit for respectively calculating a level of coincidence between the characteristic partial image of each search-target image detected with the characteristic partial image detection unit and the partial image of the input image, and searching for a search-target image corresponding to the input image from among a plurality of search-target images based on the coincidence level.