PURPOSE: To reduce the nonlinear distortion or to decrease the occupied area of a post filter by sending an output to the post filter by a time set by the transient response.

CONSTITUTION: A changeover switch S is provided between the output terminal of an operational amplifier A and the input terminal of an integration circuit B. The changeover switch S is closed with a delay time settled by the transient response of the amplifier A after a switch S1 selects the inverting input terminal of the amplifier A and closed just before the switch S1 is opened to the position of ground potential. Through the operation above, the electric charge supplied to a capacitor CF is not affected by the difference in the positive/negative response speed of the amplifier A. Moreover, wiring is applied to ground potential applied to the capacitor CF without having a common wiring resistance with switches S1∼Sn, SI so as to prevent the effect of asymmetrical fluctuation in terms of polarity of the ground potential onto the capacitor CF.