A relay agent device disposed in one of a plurality of networks each accommodating a terminal and an address leasing device performing an address lease service for the terminal, has a notifying unit notifying a proxy address leasing device, employed in common between the plurality of networks, of an address lease state of the address leasing device with respect to the terminal, a detecting unit detecting a fault in the address leasing device, and a transfer unit transferring a message related to the address lease, which is sent from the terminal, to the proxy address leasing device in order for the proxy address leasing device to take over an IP address lease service provided by the address leasing device when the fault is detected, and transferring a message related to the address lease, which is addressed to the terminal from the proxy address leasing device, to the terminal.