A communication band is divided into: a beacon period in which every master station transmits a beacon packet in competition with one another; a guaranteed-band period (e.g., a TDMA period or an FDMA period) in which only a specific permitted station is allowed access; and a CSMA period in which every station is allowed access in competition with one another. Plural master stations exchange information of a communication band used in the guaranteed-band period with each other, and based on the respective information, a communication band available to the master station's own communications system in the guaranteed-band period is calculated.