The present invention has an object to provide 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. An RN (1508) sets a TA of an eNB to be connected with the own device as a TA of the own device. For example, when the RN (1508) moves along an arrow (1500), the TA of the RN (1508) is changed from a first TA (1601) to which a fourth eNB (1525) belongs to a third TA (1602) to which a tenth eNB (1505) belongs. Upon change of the TA of the RN (1508) as described above, a user equipment device (UE; 1509) transmits a TA update request signal of the own device to a target second MME (1501).