A terminal device includes a connection environment check unit, a determination purpose port mapping command unit which issues a port mapping command to a lower layer NAT device, and a determination packet generation unit which generates a determination packet which is source-routed. Additionally, a determination packet transmitting unit transmits the determination packet, a determination packet receiving unit receives the determination packet transmitted from the determination packet transmitting unit, passing through the upper layer NAT device a and the lower layer NAT device a and arriving at the terminal device a, and a port number obtainment unit obtains a WAN-side port number of the lower layer NAT device a indicated in the received determination packet.