A mobile terminal device in a mobile communication system comprising at least one information terminal device, the mobile terminal device, and an external gateway device (21, 23), the mobile terminal device comprising: a request unit configured to perform a request for establishing only one Packet Data Network PDN connection which provides connectivity to a PDN associated with an access point name APN, the PDN connection being used only for a communication between the external gateway device and a plurality of information terminal devices which includes the at least one information terminal device; a reception unit configured to receive the APN, an evolved packet system EPS bearer ID, and an address block configured as a set of a plurality of addresses allocated for identifying the plurality of information terminal devices, the address block being allocated by the external gateway device to the PDN connection established by the request unit and being stored in association with the PDN connection by a PDN connection holding unit (214, 231) of the external gateway device, and the APN and the EPS bearer ID being allocated to the PDN connection; and a router unit configured to receive, based on the EPS bearer ID, user data from the PDN associated with the APN via the PDN connection and send the received user data to an information terminal device, the user data being addressed to an IP address that is generated from the address block and is configured in the information terminal device, and the router unit being configured to receive, based on the EPS bearer ID, user data from an information terminal device and send, based on the EPS bearer ID, the received user data to the PDN associated with the APN via the PDN connection, using the IP address as the source IP address. a request unit configured to perform a request for establishing only one Packet Data Network PDN connection which provides connectivity to a PDN associated with an access point name APN, the PDN connection being used only for a communication between the external gateway device and a plurality of information terminal devices which includes the at least one information terminal device; a reception unit configured to receive the APN, an evolved packet system EPS bearer ID, and an address block configured as a set of a plurality of addresses allocated for identifying the plurality of information terminal devices, the address block being allocated by the external gateway device to the PDN connection established by the request unit and being stored in association with the PDN connection by a PDN connection holding unit (214, 231) of the external gateway device, and the APN and the EPS bearer ID being allocated to the PDN connection; and a router unit configured to receive, based on the EPS bearer ID, user data from the PDN associated with the APN via the PDN connection and send the received user data to an information terminal device, the user data being addressed to an IP address that is generated from the address block and is configured in the information terminal device, and the router unit being configured to receive, based on the EPS bearer ID, user data from an information terminal device and send, based on the EPS bearer ID, the received user data to the PDN associated with the APN via the PDN connection, using the IP address as the source IP address.