A multimedia gateway that has an ISIM is provided. The multimedia gateway comprises: request receiving means for receiving, from a client terminal, a request message that specifies a destination IMS AS and a communications protocol, and identifying an IMPU assigned to the client terminal, establishing means for establishing a session for communication with the IMS AS by means of the communications protocol, and establishing a connection with the IMS AS over the session, authentication information sending means for sending authentication information derived from the ISIM that contains the identified IMPU to the IMS AS, request sending means for sending the request message together with the identified IMPU to the IMS AS through the connection, response receiving means for receiving a response message from the IMS AS through the connection as a response to the request message, and response sending means for sending the response message to the client terminal.