PURPOSE: To simplify the operation of a communications channel allocating function and a controller by transmitting, from a calling station to a station to be called, a signal which a destination earth station can discriminate by means of the interruption form of a carrier and selecting one of unoccupied communications channel based on an information map and using it for communications between the calling station and the called station.

CONSTITUTION: The information map is prepared by which the using circumstances of communications line in circulating, by means of a carrier detecting means, the detection of the existence and absence of the carrier to all communications lines shared with many earth stations. The signal, which the destination earth stations can discriminate from the interruption form of the carrier, is transmitted from the calling station through a CSC (common line signal channel) to the called station. The called station also informs, through the CSC, the calling station of a confirmation that it is the called station, mutually confirms a communications request and a response to the request, and simultaneously, selects one of the unoccupied communications channels by an unoccupied channel selecting means based on the information map held in a storing means and prepares it between the calling station and the called station. Thus, the communications channel allocating function and a device to control the function can be simplified.