PURPOSE: To reduce noise between a well region and a capacitor by forming a capacitor being a component of the switched capacitor filter onto the well region formed in a semiconductor substrate and connecting the well region to a noninverting input terminal of an operational amplifier.

CONSTITUTION: Capacitors CS, CF are formed on a well region formed in a semiconductor substrate and the well region is connected to a noninverting input terminal of an operational amplifier 1. That is, one electrode of a capacitance C1 between the well region and a polycrystal silicon layer is connected to the noninverting input terminal of the operational amplifier 1. Since the well region and the noninverting input terminal of the operational amplifier 1 are connected in the switched capacitor amplifier as above, the AC potential difference between them is zero. As a result, it is prevented that noise appears at an output terminal 3 via the capacitance between the well region and the polycrystal silicon layer.