When an authentication identifier is contained in a first message required for receiving authentication with respect to IEEE 802.1x from a terminal device, the authentication is assumed to be successful and then a data link is established with the terminal device, and a filtering unit is set so as to pass a first packet which should be passed for receiving authentication with respect to a higher layer protocol from among packets sent from the terminal device through the data link and to block a second packet different from the first packet. If a second message required for receiving authentication with respect to the higher layer protocol from the terminal device is received through the filtering unit, the authentication with respect to the higher layer protocol is performed. When the authentication is successful, the filtering unit is set so as to pass the second packet.