1. An image search apparatus comprising: a search server, the search server comprising: an image storing unit storing a plurality of images; an approximate-data storing unit including a memory storing approximate data of the image, in association with the image; and a processor comprising: an approximate-space searching unit that searches approximate data of a search key from the approximate data stored in the approximate-data storing unit and which outputs M pieces (M: natural number) of approximate data similar to the approximate data of the search key; and a calculating unit that calculates similarity between the search key and M pieces of images stored in the image storing unit corresponding to the M pieces of approximate data, and that outputs top K (K<M) pieces of distances as a search result, wherein the calculating unit comprises a real-space final ranking unit that performs a final ranking process on top M pieces in which the similarity obtained by the approximate-space searching unit is high and of which, rankings determined by respective approximate distances are high, and that outputs, as the search result, top K pieces (K<M) of which rankings determined by respective real distances are high, wherein a value of M is changed according to a factor of distortion due to approximation of the image, and wherein the factor of distortion due to approximation of the image indicates a degree at which a result obtained by the approximate-space searching unit is changed by the final ranking process.