1. An information retrieval device which retrieves from among information items each having a plurality of feature parameters an information item having a feature parameter similar to a specified feature parameter, comprising: a storage device storing an index which includes a plurality of concatenated keys and pointer information and retains a dictionary type order of the concatenated keys, each concatenated key being produced by discretizing a distance from each of a plurality of reference points to an object point representing information to be retrieved in a multidimensional space consisting of a plurality of dimensions corresponding to the plurality of feature parameters and by concatenating resulting discrete distances, the pointer information allowing access to information corresponding to the object point; a retrieval device determining a set of object points within a predetermined distance from a point specified in the multidimensional space and acquiring information corresponding to the object points in the set through the use of the index; and an output device outputting the acquired information.