A DHCP/authentication server transmits an IP address and authentication information acquired from the IP address to a home gateway. The home gateway creates authentication data from the authentication information and adds the authentication data to an IP packet received from a terminal, and transfers the IP packet to a false-address checking server. The false-address checking server extracts a source IP address and the authentication data from the IP packet, and creates provisional authentication data based on the source IP address. The false-address checking server checks the provisional authentication data against the original authentication data. If these two pieces of the authentication data coincide with each other, the false-address checking server transfers the IP packet to a communication counter part. If the authentication data do not coincide with each other, the false-address checking server discards the IP packet.