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 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 moves along an arrow, the TA of the RN is changed from a first TA to which a fourth eNB belongs to a third TA to which a tenth eNB belongs. Upon change of the TA of the RN as described above, a user equipment device transmits a TA update request signal of the own device to a target second MME.