Disclosed herein is a ranging method for a mobile communication system based on an OFDMA (Orthogonal Frequency Division Multiple Access) scheme. In the communication system for classifying a ranging procedure between a transmission side and reception sides into an initial ranging process, a bandwidth request ranging process, and a periodic ranging process, the ranging method includes the steps of determining the number of initial RCs(ranging codes) for the initial ranging process, the number of bandwidth request RCs for the bandwidth request ranging process, and the number of periodic RCs for the periodic ranging process, determining a backoff value of the periodic RCs according to the number of the periodic RCs, and sending the initial RCs, the bandwidth request RCs, the periodic RCs, and the backoff value of the periodic RCs to the reception sides.