PURPOSE: To make the level of an output signal constant even if ambient temperature is changed and to improve the detection efficiency by connecting a diode of a prescribed characteristic to a diode load used for a detection circuit and providing a correction circuit of the same constitution as the detection circuit.

CONSTITUTION: A diode 5 having a larger reverse resistance more than that of a diode 4 is connected in the forward direction to the diode 4 as a load of the diode 4 used for the detection circuit. Thus, the reverse current flowing to the diode 4 is blocked by the diode 5 to improve the detection efficiency. Furthermore, since the correction circuit of the same constitution as the detection circuit, even if the detection output is changed due to temperature change, a reference signal from a terminal 3 is changed similarly. Thus, the level of the output signal of an operational amplifier 8 is made constant regardless that the ambient temperature changes.