A mobile communication system capable of performing communication between a user equipment device connected to a relay device and a core network if the relay device moves. A relay device sets a tracking area of a base station device to be connected with the own device as a tracking area of the own device. For example, when the relay device moves along an arrow, the tracking area of the relay device is changed from a first tracking area to which a fourth base station device belongs to a third tracking area to which a tenth base station device belongs. Upon change of the tracking area of the relay device as described above, a user equipment device transmits a tracking area update request signal of the own device to a target second mobility management entity.