In a mobile communication system using a mobile IP, a mobile terminal determines a main home agent and a main home address. When the mobile terminal moved to a network different in service form, a home address for use on a moved-in domain network is registered as a primary care of address to the main home agent, thereby enabling communications by using the main home address even upon accessed from the other than the belonging domain network. This makes it possible to flexibly cope with an incoming call from the external by the use of a representative home address even on the Internet as a distributed network.