PURPOSE: To decrease the power consumption of an intermediate repeater and to increase the transmission capacity of a subsignal by giving a main digital signal subjected to phase modulation as a subsignal transmitted and received by an intermediate repeater.

CONSTITUTION: Terminal station repeaters 11, 12 and the intermediate repeaters 21∼23 send some service signals such as an ordering signal and a supervisory control signal in addition to a main digital signal 3. As the service signal, a subsignal 4 comprising channels CH1∼CH3 to be sent/received by the intermediate repeaters 21∼23 such as an ordering signal and a subsignal 5 comprising channels CH4∼CH7 not required to be sent/received by the intermediate repeaters 21∼23 such as a line switching command signal are given. Phase MODEMs 61, 62, 71∼73 use the subsignal 4 to apply phase-modulation to the main digital signal 3 and send the result and the subsignal is demodulated from the main digital signal 3 subjected to phase modulation to receive the sub signal 4.