PURPOSE: To reduce the effect due to a parasitic capacitor of a switch by storing an electric charge of opposite polarity to a dummy switch so as to cancel the electric charge stored in a parasitic capacitor of a switch group at read of a signal of the capacitor mutually.

CONSTITUTION: A voltage VC(nT) at an output terminal C is expressed in equation I, where V(n-1/2)T is a signal voltage inputted to an input terminal A at a phase 1, -V(n-1/2)T is a signal voltage inputted to an input terminal B, and C33, C41, C42...C48 are capacitances of capacitors 31, 41, 42...48 respectively. The equation I is expressed in equation II when same type of switches are used for switches 21, 22...24, where voltages VC, V are respectively a function of time (t), a discrete function using the phase 2(1) corresponding to a period T and VC(nT) is a voltage appearing at the output terminal C at a point of time t=nT (where; n is an integral number), thereby decreasing the effect of the parasitic capacitor of the switches.