A bidirectional control/supervisory system for transmitting control signals from a controller to devices and transmitting supervisory signals from sensors monitoring the devices to the controller through a common transmission line. A transmission/reception control circuit generates a timing signal and a supply voltage of constant level, and converts the voltage into clock pulses having voltage levels differing from the supply voltage. The clock pulses are outputted to the common transmission line under the timing of the timing signals. A first unit group is connected to the controller and the transmission/reception control circuit through the transmission line and includes first transmitter units connected thereto for modulating the level of the clock pulses with control data supplied from the controller and first receiver units connected to the controller and to the transmission/reception control circuit through the transmission line for extracting supervisory signals of the sensors to supply them to the controller. A second group is connected to the transmission/reception control circuit and includes second receiver units connected to the sensors and the common transmission line for demodulating the level of the clock pulses to extract the control data to be supplied to the devices and second transmitter units connected to the sensors and the common line for modulating the levels of the clock pulses with supervisory data signals supplied from the sensors for transmission thereof to the first receiver units.