4. The contents retrieval device according to claim 3 , further comprising a related keyword selection unit configured to select, from the contents database, the narrowing-down keyword and the expansion keyword, respectively corresponding to the narrowing-down document space information and the expansion document space information, such that a candidate keyword indicating a larger degree of relevance to the contents retrieved by said content retrieval unit is more likely to be selected by said related keyword selection unit, wherein said display unit is configured to display, as the related keyword, the narrowing-down keyword and the expansion keyword, and to display an attribute of the contents retrieved by said content retrieval unit.
5. The contents retrieval device according to claim 4 , wherein said document space storage unit is configured to store a respective item of the plurality of items of document space information for each section of the plurality of candidate keywords categorized using a DF (Document Frequency) value as the respective keyword occurrence frequency.