User authentication system and user authentication method

A subscriber's terminal 201 is configured to be connected to an internet service provided 208 via a LAN 202, and an IP subnet distribution switch 209 within a network service provider 203. When a packet signal that has not received the authentication is input, a physical-port changeover switch 204 gives a temporary IP address to the subscriber's terminal 201, and causes the subscriber's terminal 201 to employ this for making an authentication process. If the authentication succeeds, a normal IP address is given, and the packet signal, which made use of this, is distributed to a network that is an object by the IP subnet distribution switch 209.