PURPOSE: To improve the utilizing efficiency of a channel, by causing a host station to broadcast vacant channel information to plural slave stations at every carrier and update the broadcasting content in Round Robin form and the slave stations to search a carrier number and channel number and obtain an accessible channel.

CONSTITUTION: A base station 10 manages channels and always recognizes their occupied and vacant states. The base station 10 periodically searches a vacant channel and broadcasts one vacant channel information at every carrier by putting the information in the control channel (CNTL) of down signals. All subscriber's stations 20 always monitor the CNTL in the down signals and, when they require a new channel, check the vacant channel information whether a channel is usable or not and if usable request the base station 10 to use a usable channel. Upon receiving a vacant channel using request from one of the subscriber's siations 20, the base station 10 informs the subscriber's station 20 that the station 10 can use the channel by utilizing the down signals. Therefore, the utilizing efficiency of a channel can be improved.