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.
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. 14. The contents retrieval device according to claim 4 , wherein, using an associative dictionary indicating a large degree of relevance to the contents retrieved by said content retrieval unit and indicating relevance between each candidate keyword, said related keyword selection unit is 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 relevance with the search keyword is more likely to be selected by said related keyword selection unit.