Provided is a carrier allocation method in a multi cell OFDMA system capable of improving the system capacity and the bit error rate performance. In the method, a user terminal U which has received broadcast information (S101) uses an empty carrier to transmit a known symbol to a base station device (S103). The base station device estimates an average channel gain of an empty carrier block by using the received known symbol (S104), compares the channel gain sizes of the respective carrier blocks according to the estimated average channel gain and allocates a carrier block having a large channel gain to the user terminal (S105), and compares channel gains of the respective carriers in the carrier block allocated to the user terminal and selects a carrier having a channel gain lower than a predetermined threshold value (S106).