When an internal terminal (TM1 to TMn) replaced with another terminal due to the breakdown or the model upgrading and the DHCP function allocates a new private address for the MAC address of the terminal after replacement, the newly allocated private address is managed in association with physical ports (PP1 to PPn) in an expansion NAPT table. Then, in this state, when an IP packet is delivered from an external terminal (TM0) on the side of a WAN(2), the transmission destination address of this IP packet is converted to the private address of the internal terminal after conversion in accordance with the expansion NAPT table using the physical ports as a key.