PURPOSE: To obtain the resistance of a second resistor and to measure current accurately even if an accurate, stable, and expensive resistor is not used as a resistor for measuring current by selecting the connection state of switches and then using a known first resistor.

CONSTITUTION: A first resistor 14 whose resistance is known and a second resistor 28 for measuring current and then a circuit net consisting of first, second, and third switches 24, 16, and 26 are connected between an inverted input terminal and an output terminal of an operational amplifier 10 where a known voltage is supplied to a non-inverted input terminal. A voltage between both terminals of the second resistor 28 for measuring current is detected by a differential amplifier 18, is converted to a digital value by an A/D converter, and then is processed by a CPU 2.