A grouped data base 10 includes a plurality of sub-databases 10A, in which data have been stored, for respective ones of data sets having specific classification data ("male", "female", "annual income of 20,000,000 yen or more"). The grouped data base 10 is searched in accordance with a given search condition. Given search conditions are stored successively in a search-result data base 11. The sub-data bases 10A contained in the grouped data base 10 are reorganized so as to be in line with those given search conditions that have a high frequency of occurrence.