1. An image search apparatus comprising: a dimension reducing unit that reduces dimensions of multidimensional feature data extracted from an image, to produce approximate data; an approximate-data storing unit including a memory that stores the approximate data produced by the dimension reducing unit corresponding to the multidimensional feature data used to produce the approximate data; a search request receiving unit that receives at least an identifier which identifies multidimensional feature data of a person to be searched, as a search key; an approximate-space searching unit that calculates distances from approximate data corresponding to the search key received by the search request receiving unit, to plural approximate data stored in the approximate-data storing unit, based on search conditions, and arranges the distances in order of similarity; and a real-space final ranking unit that performs a further distance calculation of distances from the multidimensional feature data used to produce the approximate data produced by the dimension reducing unit, to corresponding data of the search key, on a group of results in which the similarity obtained by the approximate-space searching unit is high, determines final rankings, and outputs the final rankings as a search result, wherein the real-space final ranking unit performs a final ranking process on top M pieces (M: natural number) in which the similarity obtained by the approximate-space searching unit is high, and outputs top K pieces (K<M) thereof as the search result.