1. A document classification apparatus comprising: a feature word extraction unit configured to extract feature words from documents included in a document set; a feature word clustering unit configured to cluster the extracted feature words into a plurality of clusters 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 a thesaurus having a tree structure; a document classification unit configured to classify 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; a classification label assignment unit configured to assign a classification label to each cluster, the classification label being a word representative of the feature words belonging to the each cluster; and a presentation unit configured to present a document classification result in association with the classification label assigned to the corresponding cluster.