A mobile communication system which is capable of supporting multiple simultaneous communications on a single mobile terminal device, and collectively controlling all or a part of call states on a mobile terminal device. The system includes at least one mobile terminal device having: more than one mobile terminal side call control units for controlling calls at a mobile terminal side; and a mobile terminal side mobile terminal management unit for managing a communication state of the mobile terminal device while identifying each mobile terminal side call control unit which is currently in communication. The system also includes a network device having: a plurality of network side call control units for controlling calls at a network side; and a network side mobile terminal management unit, provided in correspondence to the mobile terminal device, for managing the communication state of the mobile terminal device and correspondences between said plurality of network side call control units and the mobile terminal device while identifying each network side call control unit which is currently in communication.