PURPOSE: To make a photodetector small-sized and connection work easy, by constituting the photodetector including an image sensor separately with a controller main body, and minimizing the number of signal lines in cables tying the body.

CONSTITUTION: A control pulse signal transmitted from a controller main body (not shown) is applied to a cable terminal 40 of a photodetector as shown in the figure, and driving signals for various circuits such as the image sensor 58 are formed. The output signal of the image sensor 58 is transmitted to the contoller main body, this output signal is picked up separately from the other signals and a display signal representing the detection range of the image sensor is formed based on this signal. The controller main body and the photodetector are connected with two (a pair) signal lines only, and control pulses, display signals and image sensor output signals are superimposed on the two lines.