1. A contents retrieval device which displays a related keyword that is related to a search keyword for retrieving contents of a plurality of contents stored in a contents database, each content of the plurality of contents being stored in the contents database with a plurality of candidate 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 of the plurality of contents, from the contents database, according to the acquired search keyword; a document space storage unit including a memory in which a plurality of items of document space information are stored, each respective item of the plurality of items of document space information indicating a degree of relevance between (i) each content of the plurality of contents stored in the contents database and (ii) respective candidate keywords of the plurality of candidate keywords, each respective candidate keyword of the plurality of candidate keywords being categorized and associated with a respective item of the plurality of items of document space information according to a respective keyword occurrence frequency of the respective candidate keyword, the respective keyword occurrence frequency indicating, for the respective candidate keyword, a number of contents of the plurality of contents in which the respective candidate keyword appears, and the plurality of items of document space information including first document information being associated with a respective keyword occurrence frequency and the respective keyword occurrence frequency associated with the first document information indicating a degree of relevance between the search keyword and each of the plurality of contents; a document space selection unit configured to select, from among the plurality of items of the document space information, (i) one item of the document space information, as second document space information, the second document space information being associated with a respective keyword occurrence frequency that is less than the respective keyword occurrence frequency associated with the first document space information, and the second document space information being associated with a respective candidate keyword, of the plurality of candidate keywords, that narrows down a number of contents, of the plurality of contents, to be retrieved by said content retrieval unit, and (ii) one item of the document space information, as third document space information, the third document space information being associated with a respective keyword occurrence frequency that is greater than the respective keyword occurrence frequency associated with the first document space information, and the third document space information being associated with a respective candidate keyword, of the plurality of candidate keywords, that expands a number of contents, of the plurality of contents, to be retrieved by said content retrieval unit; and a display unit configured to display, as the related keyword, each candidate keyword of the plurality of candidate keywords that satisfies a predetermined standard in the second document space information and the third document space information selected by said document space selection unit.