A clustering system that clusters a language model group includes a union language model preparation unit that prepares a union language model for each language model so as to include a union of vocabularies in the language model group as entries, and a clustering unit that performs clustering with respect to the union language model group so as to classify the union language model group into a plurality of clusters. When the union language model preparation unit prepares a union language model for a certain language model, the union language model preparation unit records, regarding vocabularies included in the certain language model as a basis, occurrence frequencies of the corresponding entries in the certain language model, and records, regarding vocabularies not included in the certain language model, data showing that an occurrence frequency is 0. Thereby, a clustering system capable of clustering language models that includes voice uttered by or text written by a plurality of speakers can be provided.