PURPOSE: To improve transmission delay characteristics by utilizing a tone signal at a different frequency as a request signal.

CONSTITUTION: Frequencies f1-f4 and the tone signal for request with a slot number (k) are assigned to miniaturized earth stations 3-1-3-4 and when a communication request is generated from a miniaturized earth station, the assignation of a communication channel is requested through a communication satellite 1 to a central earth station 2 by using this tone signal. Since the frequency of the tone signal from the station 3 is different, even when the plural statuons 3 simultaneously transmits the request signals to the station 2, the tone signals can be separated for each frequency by the receiver of the station 2. The station 3 receives TDM signals from the station 2 at all times, only the information signal of a TDM slot containing the ID signal of the its own station is detected by using a transmitter/receiver provided at the station concerned, transmission timing is recognized from the communication slot number assigned to the station concerned, and the transmission of information from the stations 3 is started. Thus, the plural stations 3 can simultaneously transmit the request signals to the station 2, and the transmission delay characteristics can be considerably improved.