1. A contents retrieval device which displays a related keyword related to a search keyword used for retrieving contents based on a contents database in which each of a plurality of contents is stored with a plurality of keywords, said contents retrieval device comprising: a keyword acquisition unit configured to acquire the search keyword; a content retrieval unit configured to retrieve the contents according to the search keyword from the contents database; a document space storage unit in which document space information is stored, the document space information indicating a degree of relevance between each contents and each keyword, and the document space information being categorized according to an occurrence frequency indicating occurrence of each keyword included in the contents database appears, with respect to each contents in the contents database; a document space selection unit configured to select (i) second document space information including the keyword used for narrowing down the number of contents to be retrieved and (ii) third document space information used for expanding the number of contents to be retrieved, according to a magnitude relationship of the occurrence frequency of the document space information stored in said document space storage unit, using, as a basis, an occurrence frequency of the keyword in first document space information which indicates a degree of relevance between the search keyword and contents according to the search keyword; and a display unit configured to display each of keywords which satisfies a predetermined standard in the second document space information and the third document space information selected by said document space selection unit, as the related keyword.