A DT matrix generation means (11) generates a DT matrix (11A) from each document (D) in a document set (21) and each term (T) in a term list (22). A DT matrix transformation means (12) generates a transformed DT matrix (11B) by performing DM decomposition of the DT matrix (11A). A document classification means (13) extracts and outputs, for each cluster appearing on the transformed DT matrix (11B), each document (D) belonging to the cluster as one classification (subset).