In an authentication server, when a mobile terminal is connected with a mobile network constructed by an IP network, a network-connection authentication of the mobile terminal is performed. A setup of a path from the mobile terminal to a home agent according to a mobile IP is performed after the network-connection authentication is performed. A mobile-management-service authentication of the mobile terminal with the home agent is performed after the setup of the path is performed. The authentication server includes a registration part which is configured to register an IP address of the mobile terminal and a domain name of the mobile terminal to a domain managing server which manages IP addresses and domain names.