A client transmits an event of Event::Subscribe ("DSI_—CHANGED") to a server. The server returns an event number set for a "DSI_—CHANGED" event to the client. After the event number is obtained, the client transmits Event::notify to the server and requests some event generation notice including "DSI_—CHANGED". When there is a change in DSI of a karrusel which was received, namely, when the "DSI CHANGED" event is generated at a certain timing after Event::notify is received, the server transmits the event number corresponding to the "DSI_—CHANGED" event to the client as a response to Event::notify. The contents of the karrusel before the event number is generated is continuously used for a period of time during which the client transmits Session::detach to the server after the server notified the client of the generation of the event number.