PURPOSE: To reduce the number of parts by operating a gate means with the aid of a pulse obtained by modulating pulse width outputted from a pulse width modulation means, permitting a filter circuit to amplify an input voltage fetched from the gate means and regarding it as an output voltage.

CONSTITUTION: If a duty ratio setting means 10 is set so as to generate a reference voltage Vref, a comparator 22 in the pulse width modulation circuit 20 modulates a triangular wave W with a period T into a pulse P with a pulse width tON due to the level of the voltage Vref, changes over a switch 30 with the aid of the pulse P and drives an active filter circuit 40. It converts a pulse string composed of an input voltage Vi and a peak value Vi at a ground level into a current satisfying V01 =D.Vi according to the duty ratio D, simultaneously gives K-fold gains, and outputs an output voltage satisfying V0=K.D.Vi. Thus the gain can be made highly accurate.