PURPOSE: To detect current change highly accurately regardless of manufacturing errors in resistance values, by providing a correcting circuit constituted by a pair of transistors forming a current mirror circuit and a resistor in a circuit for generating an input voltage to be detected.

CONSTITUTION: A correcting circuit, which is composed of transistors Q3 and Q4 constituting a current mirror circuit and a resistor R4, is provided in a circuit for generating an input voltage to be detected, which is composed of transistors Q1 and Q2 and resistors R1WR3. In this constitution, the input voltage to be detected Vin is proportional to the ratio between the resistor R3 for generating the input voltage to be detected R3 and the resistor R1 for adjusting the collector current of the transistor Q3. Therefore the input voltage to be detected Vin does not depend on the value of the resistor R3, whose manufacturing errors are large, just like a reference voltage Vref. The input voltage Vin is determined by a resistance ratio (R3/R4), which hardly yields errors.