A retrieval information that is a retrieval target, acquired from an information source, is arranged on a vector space. Data of a usage information and a content specifying manners of usage, used by the user in the past is acquired by a usage-information acquisition unit. A user's preference extraction unit extracts a preference of a user from the data, and notifies a number-of-effective-elements reduction unit of the extracted data. The number-of-effective-elements reduction unit evaluates each element of a vector of retrieval information by using the preference of the user, and reduces the number of effective elements by removing the elements smaller than a certain criteria. Thereafter, a statistic is acquired in the vector space of the vector of the retrieval information, and the cosine normalization is performed on the result that includes a large number of elements, and the normalization in which the variation of the statistic was compensated by the average weight is performed on the result that includes a small number of elements.