Abstract: There are provided a main master station for transmitting control data by setting, as a synchronization slot, a specific transmission slot of a time division multiple access communication frame, a sub master station for transmitting the control data by setting, as a broadcast slot, a slot which does not overlap with the synchronization slot in synchronization with the main master station, and a slave station capable of communicating with the main master station and the sub master station, and the slave station waits for the control data transmitted from the sub master station in a time width decided based on a time position of the synchronization slot upon receipt of the control data from the main master station, and carries out an incoming call operation while continuously receiving the control data from either the main master station or the sub master station in an incoming call received from the other.