A plurality of database hosts hold subscriber information in a distributed manner. A retrieval server processes inquires (host retrieval requests) on which of the database hosts has desired subscriber information stored therein. The retrieval server includes a plurality of processors, and the plurality of processors performs processing in parallel. Therefore, inquiries can be processed at a high speed. Further, a conversion information database and a host information database are provided as in-memory databases, which allow inquiry processes to be processed at a high speed. A call control server can efficiently acquire subscriber information from the database host based on the result of an inquiry made to the retrieval server.