1. A classification hierarchy regeneration system comprising: a clustering unit which clusters a data group associated with a hierarchical classification, and generating a classification group, i.e., a group obtained by extracting a classification satisfying a condition defined in advance from classifications corresponding to respective data in a cluster; a cooccurrence degree calculation unit which calculates a degree of cooccurrence of two classifications selected from the classification group; and a classification hierarchy regeneration unit which regenerates a hierarchy of the classification, based on the classification group and the degree of cooccurrence.