1. A content data retrieval apparatus which retrieves content data associated with one or more attribute values, said apparatus comprising: an attribute value storage unit configured to store the one or more attribute values, the one or more attribute values each indicating a content attribute, and the one or more attribute values being distributed in an attribute value space having a predetermined size; a receiving unit configured to receive a search word as an input; a conversion table in which the search word is associated with a search word range, the search word range having a range of the one or more attribute values associated with the search word, the range of the one or more attribute values having a distribution of the range of the one or more attribute values, the search word range having a relative relationship between the search word range and the distribution of the range of the one or more attribute values, and the search word range being a two- or more than two-dimensional space range; and a processor including a modification unit configured to modify the relative relationship between the search word range and the distribution of the range of the one or more attribute values by modifying the range of the one or more attribute values or by modifying the search word range in accordance with the distribution of the range of the one or more attribute values stored in said attribute value storage unit, wherein said modification unit is further configured to modify the search word range by reducing a size of the search word range such that a ratio in size of the distribution of the range of the one or more attribute values to an entirety of the attribute value space becomes equal to a ratio in the size of the search word range to the distribution of the range of the one or more attribute values, and by moving the reduced size search word range such that the reduced size search word range is included in the distribution of the range of the one or more attribute values, and a retrieval unit configured to retrieve the content data corresponding to the search word by searching the range of the one or more attribute values associated with the search word range in the relative relationship between the search word range and the distribution of the range of the one or more attribute values modified by said modification unit, the search word range being associated with the search word received by said receiving unit and being modified by said modification unit.
7. 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 exceeds a predetermined number, said modification unit is further configured to modify the search word range by reducing the size of the search word range so that the number of the one or more attribute values included in the search word range is the predetermined number or below.