In a case where internal links connected to plural mobile routers in a mobile network respectively have unique network identifiers initially set, the mobile router, when receiving a network identifier different from the initial setting advertised from another mobile router, selects either one of the network identifier initially set to the self and that advertised, as a network identifier of the internal link connected to the self. Then, if the one selected is identical to that initially set, the mobile router advertises the network identifier to within the mobile network and registers it in a home agent. If not identical, the mobile router registers the one advertised in the home agent as a network identifier of the internal link connected to the self.