PURPOSE: To cancel the offset voltage of an operational amplifier and to extract an output continuously by providing two switched capacitor circuits so as to operate complementarily, and an output selection circuit selecting the output at a prescribed period mutually and extracting the result.

CONSTITUTION: Two switched capacitor circuits short-circuiting an inverting input terminal and the output terminal of operational amplifiers 1a, 1b for a 1st period to cancel the offset voltage of the operational amplifiers 1a, 1b and obtaining the output signal with the offset voltage cancelled out therein for a 2nd period, are operated complementarily. Then an output selection circuit 40 is provided, which selectively extracts the output for the 2nd period of the switched capacitor circuits 20, 30 for a prescribed period. Thus, the offset voltage of the operational amplifiers is cancelled and the output signal is sampled in any timing continuously by the circuit of the next stage.