A client for IPv4 having a SIP function sends a first REGISTER message to an adapter. Then, the adapter executes an authentication sequence of EAP-AKA for an access gateway connected between an IMS/MMD network and an IPv4 network, then establishes a tunnel connection. Then, the adapter generates a second REGISTER message corresponding to IPv6 based on the first REGISTER message corresponding to IPv4. The adapter sends the second REGISTER message to a SIP server connected to the IMS/MMD network through the tunnel connection to the access gateway.