A relay server that implements a virtual file sharing system includes shared resource and message information that can be managed in an integrated manner, and a user of a client terminal can be appropriately notified of changes of the shared resource, or the like. The relay server includes a shared resource information registration unit, which stores shared resource information when a resource is shared by a plurality of client terminals. The shared resource information includes, for example, resource information, account information of the client terminal that shares the resource, and message information regarding a message exchanged among the client terminals. The relay server includes a control unit, which adds, when the resource becomes sharable or is shared by the client terminal, message information regarding a message notifying of such resource sharability or resource sharing to the shared resource information.