PURPOSE: To prevent the influence of temperature by changing the reference voltage of an amplitude identifying circuit in parallel with the compared input signal voltage at the same temperature coefficient.

CONSTITUTION: The non-inversion input terminal of an operational amplifier OP is connected to a connecting point of a series circuit of a resistance R1 and a variable resistance RV. This series circuit is used as a voltage adjusting circuit, and inversion input terminal of the operational amplifier OP is connected to output of the operational amplifier OP, and the output terminal of the operational amplifier OP is connected to a terminal of plural diodes D1∼Dm connected in series. Another terminal of diodes D1∼Dm is connected to the ground through a resistance R2, and the voltage is made to have the same characteristic with temperature characteristic of the input signal voltage by using the output terminal as a temperature compensation output terminal.