In subscriber units 0 to (N−1), a subscriber unit to be allocated to a bandwidth is defined as a subscriber unit K, where K is an integer from 0 to (N−1) inclusive. In a bandwidth allocating method, a free bandwidth in a communication channel ch(K−1) not used by the subscriber unit (K−1) is found, and a bandwidth requested by the subscriber unit K is allocated to the free bandwidth in the communication channel ch(K−1). A free bandwidth in the communication channel chK is found on the basis of a residual bandwidth requested by the subscriber unit K not allocated to the bandwidth in the communication channel ch(K−1), and the residual bandwidth requested by the subscriber unit K is allocated to the found free bandwidth in the communication channel chK. The operation is repeated for the subscriber units 0 to (K−1) and (K+1) to (N−1).