PURPOSE: To eliminate the effect of an offset voltage with simple constitution by constituting the circuit that a capacitor for differential voltage detection is charged by a voltage value between any input voltage and an offset voltage to any input voltage.

CONSTITUTION: Switches S1, S2 are interlocked so that the states as shown in the figures (A), (B) are repeated alternately. Thus, an electric charge of C1. (V2-Vofs) is stored in a capacitor CD for detecting a voltage difference at the period of (A), where Vofs is the offset voltage of an operational amplifier OP. Further, an electric charge of C1.(V1-Vofs) is stored in the capacitor CD for differential voltage detection at a period (B) and the electric charge difference C1.(V2-V1) between the periods is moved to the capacitor CI for difference voltage integration. As a result, the difference voltage of (C1/C2).(V2-V1) is stored in the difference voltage integration capacitor CI every switching of the switch and the accumulated voltage does not include the error component by the offset voltage Vofs at all.