Unauthorized access from a network to a terminal is prevented. If a signaling packet received from an external terminal contains a virus, the signaling packet is discarded. If the signaling packet does not apparently contains a virus, an unnecessary part is deleted and the signaling packet is transferred to an internal terminal in response to a polling signal. A port number described in a response packet received from the internal terminal is translated to a different port number. If a session packet received from the external terminal does not conform to a predetermined protocol, the session packet is discarded. If the session packet received from the external terminal conforms to the predetermined protocol, a port number is subject to reverse translation and a resultant session packet is transferred to the internal terminal.