The A/D converting circuit 20 is provided with a differential amplifying portion 21, a first variable capacitance portion 22A, a second variable capacitance portion 22B, a comparing portion 23, a connection controlling portion 24, a first feedback portion 25A and a second feedback portion 25B. Voltage values output as a differential signal from the first output terminal and the second output terminal of the differential amplifying portion 21 are converted to 6-bit digital values by a successive approximation type A/D converting circuit (made up of a first variable capacitance portion 22A, a second variable capacitance portion 22B, a comparing portion 23 and a connection controlling portion 24) and output. A difference in potential between the first common point P1 and the second common point P2 is fed back to the differential amplifying portion 21 by the first feedback portion 25A and the second feedback portion 25B, and again converted to a 6-bit digital value by the successive approximation type A/D converting circuit and output.