A search unit 11 presents a Web search result report to a client terminal 20. A theme space construction unit 12 updates theme information within a theme information DB 13 by a user-acquired information feedback from the client terminal 20, and performs a Web search to construct a theme space. The search unit 11 includes an exhaustivity calculation unit that calculates exhaustivity being a ratio of a user-acquired part to a part of an entire target theme. Moreover, the search unit 11 includes a difference calculation unit that calculates a difference between a “user-acquired” range when a user has acquired target Web pages and a user-acquired range before that, and a relevance calculation unit that calculates relevance of the target Web pages to the target theme.