1. A document clustering system comprising: a concept tree structure acquisition unit that acquires a concept tree structure that represents the hierarchical relationship of a concept of a plurality of words; a document set acquisition unit that acquires a document set, which is a collection of documents; a concept similarity computation unit that finds the concept similarity between two arbitrary words of the document set that was acquired by the document set acquisition unit, the concept similarity being an index indicating the closeness of the two words in a concept; an inter-document similarity computation unit that finds inter-document similarity, which is the degree of semantic similarity between two documents that are included in the document set that was acquired by the document set acquisition unit, based on the concept similarity found by the concept similarity computation unit; and a clustering unit that performs document clustering of the document set based on the inter-document similarity that was found by the inter-document similarity computation unit.