PURPOSE: To duplicate easily a remote supervisory line by dividing an input signal into two, transmitting one signal as it is and storing other signal to a buffer and then transmitting to a low speed line so as to use a line having a different transmission speed.

CONSTITUTION: A control station 1 scans (3) an input signal ID via an input circuit 2 and distributes it into two at a P/S converting section 4. One signal is subject to P/S conversion 4a and modulation 5a as it is and transmitted to a transmission line 7a. The other signal is stored in a full word buffer 4c, converted into a low speed by a P/S converter 4b, subject to modulation 5b and transmitted to a low speed transmission line 7b. In a station 8 to be controlled, the signal transmitted through the transmission lines 6a, 6b is subject to demodulation 10a, 10b, the transmission line is switched (9), the signal is subject to S/P conversion 11 and output information 00 is outputted from an output circuit 13 via a distributor 12. Thus, lines having different transmission speeds are used to duplicate the supervisory lines and cope with the increasing information amount. This system is suitable for the smooth operation of power systems.