PURPOSE: To reduce the error due to temp. change, by mounting a temp.-sensitive resistor element changing in a resistance value by temp. change in order to perform the temp. compensation of the light emitting power of a light emitting diode.

CONSTITUTION: When it is supposed that a current flows to the object to be detected arranged in the coil of CT 1 being a current transformer and a current of Vvolt is induced in CT 1, the current Y flowing in a light emitting diode 2 will be as given in the formula (wherein Vf is the voltage drop in the forward direction of the light emitting diode and Rx is the resistance value of a temp.- sensitive resistor element). As shown by the formula, since the resistance value Rx may be reduced in order to increase the current flowing to the light emitting diode 2, a resistor rising in temp. and reducing in its resistance value is used as the temp.-sensitive resistor element Rx in order to perform the temp. compensation of the light emitting power of the light emitting diode 2. As this temp.- sensitive resistor element Rx, for example, an NTC thermistor or the like is used.