Each radio station can manage a plurality of channels, and functions both as a master station which controls transmission in each channel and a slave station which operates under control of a master station. In case of a master station, it transmits a signal under control of the own station, and in case of a slave station, it transmits a signal under control of the master station. Each radio station can communicate with any other radio station either directly or through another radio station. In each channel, communication is carried out in time division of centralized control access phase and distributed control access phase. A master station in case of centralized control access phase is determined in each channel and in each link.