PURPOSE: To realize the high-quality transmission of a supervisory signal by performing slightly the modulation of a main digital modulation wave of subcarrier modulated by the supervisory signal.

CONSTITUTION: A data train outputted from a parallel-serial converter 151 is given band limitation on it by an LPF, supplied to an AM modulator 154 and modulate a subcarrier oscillated by a oscillator 155 to AM. The AM modulated subcarrier is supplied to the AM modulator of a relay station as a modulation signal and slightly modulate the amplitude of main digital modulation wave to be directly relayed. The amplitude modulated digital modulation wave is amplified and outputted from an antenna. A composite modulation wave which become a medium frequency after received by the receiver of a terminal station is supplied to an input terminal 41 of a demodulator, and a supervisory signal is demodulated by a detector 42 and LPF43 for eliminating a medium frequency. By this system, the high-quality transmission of a supervisory signal can be realized because almost no deterioration occurs on main digital modulation waves.