5. The content data retrieval apparatus according to claim 1 , wherein when the number of the one or more attribute values included in the search word range is less than a predetermined number, said modification unit is further configured to modify the search word range by enlarging the search word range so that the number of one or more attribute values included in the search word range is the predetermined number or above.
5. The content data retrieval apparatus according to claim 1 , wherein when the number of the one or more attribute values included in the search word range is less than a predetermined number, said modification unit is further configured to modify the search word range by enlarging the search word range so that the number of one or more attribute values included in the search word range is the predetermined number or above. 6. The content data retrieval apparatus according to claim 5 , wherein the search word range is located within the attribute value space, and wherein said modification unit is further configured to determine a ratio between a respective dimension lengths of the search word range within the attribute value space according to a ratio between a respective dimension lengths within the attribute value space of the distribution of the range of one or more attribute values, and modify the search word range by enlarging the size of the search word range such that the number of the one or more attribute values included in the search word range is a predetermined number or above, while keeping the determined ratio of lengths.