PURPOSE: To eliminate an electromagnetic hazard, by converting the light received from the end of an optical fiber into electric power, detecting the external data by a sensor by using the above electric power and converting into an electric signal, and furthermore, converting the electric signal into an optical signal.

CONSTITUTION: An optical fiber 20 is used to connect between a sensor 10 and a control circuit 30, and the light received to a light receiver 17 from the end of the fiber 20 is converted into an electric power by a light-power converter 11. A sensor 13 detects the external data by using the above electric power, the output electric signal of the sensor 13 is converted into an optical signal by a power-light converter 15, and the light is input to the end of the fiber 20. Consequently, the circuit 30 and the censor 10 can be connected by a single line of the fiber 20, so as to realize a light weight. Furthermore, since there is no electric line between the circuit 30 and the sensor 10, the possibility to receive an electromagnetic noise is also reduced.