PURPOSE: To turn ON main power supplies of a plurality of remote communication units individually or simultaneously by changing the logical state of a communication line through a first communication unit and detecting the changed logical state on the side of a second communication unit.

CONSTITUTION: When a receiver 8 in a WS41n being driven through a stand-by power supply receives a power supply control signal from an MCU 40 under a state where the main power supply of a power supply unit 14 is interrupted, output of the receiver 8 is fed through an inverter circuit 9 and an OR circuit 10 to the reset release input R of a counter 12. Since a WS41c has been provided with the power supply control signal from the MCU 40, CPCSW of a switch circuit 11 is turned ON and goes Low. When the output of the counter goes High, a power supply ON signal is fed to the power supply unit 14. Consequently, costs and power consumption of the communication system can be reduced and operability thereof is enhanced.