To eliminate influence of a noise originating in the usage environment, and to simultaneously detect a short circuit (switching) of a lead and a disconnection thereof, which is excellent in costs and reliability.

Terminal parts of two leads 1A and 1B are connected with a diode 10. The leading end parts of the two leads 1A and 1B are connected to a power source terminal 14 through respective resistors 13A and 13B, and are connected to a switching part of a mechanical or electrical switch 20 having switches 21A and 21B which complementarily switch each other. By switching the switches 21A and 21B each other, a bias voltage applied to the diode 10 is mutually switched in a forward direction and a backward direction. The disconnection state of the leads 1A and 1B in the forward direction or the short circuit state of the leads 1A and 1B in the backward direction is detected as the change in the voltage or the current.