A plurality of terminal units are connected to the central unit by one optical fiber line in a loop form, thereby forming a communication link. The central unit converts the data including the terminal address and control bit into the serial optical signal and outputs this optical signal. The terminal unit receives and interpretes the optical signal from the central unit and ON/OFF controls the solenoid valve. After completion of the control, the terminal unit rewrites the control bit in the reception data to the data bit indicative of the operating state of the valve and transmits the data bit to the next terminal unit. The central unit receives the signal from the final terminal unit and interprets the bit data which was rewritten from the control bit by the terminal unit, thereby displaying the operating state of the valve.