To reduce change on a cellular network side required at the time of utilizing a cellular network by way of an IP network.A radio terminal 110 puts a location registration message for a cellular network 100 on an IP packet, and sends the message to an access point 102. This IP packet is transferred to a control proxy server 131. The control proxy server 131 stores a correspondence between identification information on a radio terminal and an IP address based on a location registration request. If necessary, the location registration request is converted into the location registration message in compliance with a standard of the cellular network 100. The location registration message is transferred to an MSC/VLR 121 via a gateway 130. As the gateway 130 looks like an ordinary base station to the MSC/VLR 121 and an ordinary location registration message is usable, any change on the cellular network side is not necessary.