PURPOSE: To expand an operating range of an automatic threshold value adjusting circuit by preventing an output signal of an amplifier amplifying an input signal from being clamped at large signal input.

CONSTITUTION: When an input voltage VA of a comparator COMP is a VBE (where; VBE is a forward voltage drop of each diode) or below, that is, when an output voltage Vo of the amplifier AMP is ≤2VBE, the gain of the amplifier AMP is -(R0/R1). When the input voltage VA is the VBE or below, that is, the output voltage Vo of the amplifier AMP is larger than 2VBE, the feedback resistor of the amplifier is changed to R0//R3, the gain of the amplifier AMP is -(R0//R3)/R1 and then the gain of the amplifier is lowered. Thus, the amplitude range of the input signal until the upper part of the output signal VA of the amplifier AMP is clamped is expanded.