disclosed is an upstream access method in an orthogonal frequency division multiple access - based mobile communication system in which a random access method in an ofdma network includes randomly selecting , by each subscriber device , one of a plurality uplink random access channels , if a random access request to the ofdma network is transmitted by each subscriber device , accessing through the selected channel if a collision has not occurred in the selected channel , randomly selecting one of the uplink random access channels , if a collision has occurred in the selected channel , and if the number of retries for channel selection is less than a predetermined value , and determining whether a has collision has occurred , accessing through a finally selected channel , if it is determined that a collision has occurred ; and randomly selecting one of the uplink random access channels if it is determined that a collision has occurred .