An amplifying circuit amplifies and outputs voltage difference between first external input voltage and voltage corresponding to second external input voltage. An A/D converting circuit converts the output voltage of the amplifying circuit into a digital value by comparing it with reference voltage, and outputs it. A bias circuit selects the second external input voltage or voltage obtained by adding bias voltage to the second external input voltage, according to the output voltage of the amplifying circuit, and outputs it as the voltage corresponding to second external input voltage. A first storing circuit prestores a measured value of the bias voltage, and outputs the stored value as a digital value whose bit number is greater than that of the digital value from the A/D converting circuit, when the voltage corresponding to second external input voltage is the voltage obtained by adding the bias voltage to the second external input voltage.