1. An image feature extraction device for determining if a first image is a copy of a second image, the device comprising: first means for carrying out a respective set of classifying processes for each of a plurality of target regions of the first image, each classifying process generating one or more quantization indexes as a function of (a) a feature associated with the classifying process and (b) a quantization method associated with the classifying process, each set of classifying processes including at least two classifying processes carried out in an hierarchical sequence, at least one lower hierarchy classifying process being selected as a function of the quantization index generated by a higher hierarchy classifying process in the same set of classifying processes, the first means comprising: a next-hierarchy quantization method selection unit configured to, when a quantization index generated by a higher hierarchy classifying process is supplied as a feedback, select a lower hierarch classifying process corresponding to the supplied quantization index and, when a quantization index is not supplied, select a highest hierarchy classifying process for each quantization target region; a feature extraction unit configured to extract a feature used by the selected classifying process from the first image for each quantization target region; and a quantization index calculation unit configured to, for each quantization target region, quantize the extracted feature in accordance with the selected quantization method to calculate a quantization index and, in a case that the calculated quantization index is not of a lowest hierarchy, supply the quantization index to the next-hierarchy quantization method selection unit as a feedback; and second means for outputting a hierarchical quantization index code comprising a set of quantization indexes including at least one quantization index generated by a lowest hierarchy classifying process for each target region.