1. A classification hierarchy regeneration system comprising: a processor; a clustering unit which clusters a data group associated with a hierarchical classification, and generates a classification group obtained by extracting a classification which appears in a cluster with a frequency more than a number defined in advance from classifications corresponding to respective data in the cluster; a cooccurrence degree calculation unit which calculates, using the processor, 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, wherein when classifications in the generated classification group are separated in the hierarchy of the classification by at least a predefined distance, the clustering unit generates classification groups by dividing the classification group, wherein the cooccurrence degree calculation unit calculates the degree of cooccurrence, based on a cooccurrence frequency which is the number of data in which two classifications cooccur, and the number of data belonging to each classification, wherein the classification hierarchy regeneration unit determines whether the above two classifications are in inclusion relationship or in same-meaning relationship, on the basis of the degree of cooccurrence, wherein the hierarchy of classification is regenerated on the basis of a determination result indicating whether the two classifications are in inclusion relationship or in same-meaning relationship, and wherein the clustering unit generates a classification group by extracting a classification in which the number of data belonging to the classification is more than a number defined in advance from the classifications corresponding to the respective data in the cluster.