A communication system is provided that allows the same mobile terminal to be used to perform communication even if a network other than a mobile communication network is used and allows to the communication to be maintained across networks, and is capable of collectively managing charges for the networks.An information service network comprises a Mobile IP HA device, an AAAH device for managing and authenticating a mobile terminal and receiving accounting information, a VOIP device for converting voice data transferred to and from the Internet into real voice, and a service device, which may be any application server. Various networks, such as a public telephone network, mobile communication network, private IP network, and supermarket/convenience store network, have an FA device for registering the position of the mobile terminal with the HA device and an AAAL device for exchanging authentication and accounting information with the AAAH device.