The present invention relates to a mobile communication system, and more particularly, to a repeater and a ranging method using the repeater. In the mobile communication system including the repeater, a ranging operation is performed so that data are transmitted to a base station regardless of whether a mobile terminal is within a repeater area. Accordingly, it is determined whether a ranging code received through a ranging flag is directly received from the mobile terminal or if it is received through the repeater. When the ranging code is transmitted to the base station through the repeater, the ranging operation of the mobile terminal is performed.