In registering operation of a document to be searched for, a document identifier management table for managing a range of a document identifier stored for each page and a page identifier of the page is created, and an individual-search-server's search range management table for managing the range of the document identifier in charge of each search server is created. In searching operation of each search server of the document to be searched for, the individual-search-server's search range management table is referred to acquire a range of the allocated document identifier. For each index key forming a query term specified as a query condition, the document identifier management table is referred to to acquire the page identifier storing the document identifier of the allocated range. The searching operation is carried out by referring to a page shown by the acquired page identifier.