This invention aims to provide an efficient means of performing a document search wherein relevance between plural document databases is examined. A summary making module and a search module are provided to document databases, and these are connected to a network as a server. A client obtains a relevant set of documents in a specified document database via this summary from a set of documents in the specified document database. The summary obtained is sent to another server, and a search is performed according to the summary in a document database in the server to which the summary is transferred.