An image search apparatus has: a dimension reducing unit for reducing dimensions of multidimensional feature data to be produced as approximate data; an approximate-data storing unit for storing the produced approximate data with corresponding to the multidimensional feature data before the dimension reduction; a search request receiving unit for receiving an identifier which identifies multidimensional feature data of a person to be searched, at least as a search key; an approximate-space searching unit for calculating distances between approximate data corresponding to the received search key, and plural approximate data stored in the approximate-data storing unit, and for arranging the distances in order of similarity; and a real-space final ranking unit for again performing a distance calculation with using the multidimensional feature data before the dimension reduction, on a group of results in which the similarity is high, and for determining final rankings.