1. A document classification apparatus comprising: a storage device that stores at least one or more thesauruses; and a processor that performs operations, comprising: extracting feature words from documents included in a document set; clustering the extracted feature words into a plurality of clusters by using the one or more thesauruses stored in the storage device so that a difference between the number of documents each including any one of the feature words belonging to one cluster and the number of documents each including any one of the feature words belonging to another cluster is equal to or less than a predetermined reference value, the clusters corresponding respectively to subtrees of the thesaurus having a tree structure; classifying the documents included in the document set into the clusters so that each document belongs to the cluster to which the feature word included in the each document belongs; assigning a classification label to each cluster, the classification label being a word representative of the feature words belonging to the each cluster; and presenting a document classification result in association with the classification label assigned to the corresponding cluster, wherein the clustering of the extracted feature words further comprises clustering a plurality of feature words which do not each correspond to any subtree in the thesaurus, into one cluster, and the assigning of the classification label comprises assigning a classification label representing that the cluster is a set of the feature words not corresponding to any subtree of the thesaurus, to the cluster to which the feature words that do not each correspond to any subtree in the thesaurus belongs.