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; 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.