A subordinate server creates an event notice list, that is a list of the stored electronic document files having a high use frequency, and copies it onto a host server such that the host server holds it on a subordinate server-by-server basis. When an electronic document file is changed in a certain subordinate server, the subordinate server copies the changed electronic document file onto the host server, and at the same time, checks if the changed document file is present on an event notice list of another subordinate server held by the host server, and notifies a change event to the other subordinate server for storing it on a change history list, only when the file is present. If the subordinate server is requested for looking up an electronic document file, and the requested electronic document file is present on the change history list, the subordinate server copies the corresponding electronic document file from the host server for allowing the requester to look up.