1. An image retrieval apparatus, comprising: a memory unit configured to store therein a plurality of image feature values; an acquisition unit configured to acquire a first image feature value for a retrieval target image; a retrieval unit configured to retrieve an image feature value that is similar to the first image feature value from the plurality of image feature values stored in the memory unit, based on the first image feature value acquired by the acquisition unit; a determining unit configured to determine a linear conversion parameter, to be used for linear conversion out of a group of linear conversion parameters that are based on a plurality of image feature values for a plurality of images of the same photographic subject obtained under a plurality of conditions, based on the first image feature value and image feature values for said photographic subject; a conversion unit configured to convert the first image feature value by the linear conversion parameter determined by the determining unit, the conversion unit converting the image feature value retrieved by the retrieval unit using said linear conversion parameter; and an output unit configured to output conversion results of the conversion unit; wherein the plurality of image feature values are representative image feature values of respective groups of image feature values, each of the respective groups having image feature values that are similar to each other; and wherein the conversion unit configured to convert the first image feature value by the linear conversion parameter, and to convert, by said linear conversion parameter, respective image features of a group that is represented by a representative image feature value retrieved by the retrieval unit.