To improve the power factor of an AC input voltage by connecting a large-capacity flyback voltage storing capacitor to the auxiliary winding of a current feedback inductor and the connecting point to the input side and main winding through a reverse-current preventing diode.

When an AC voltage from an AC input power source 1 is supplied to the center tap of a transformer 10 through a current feedback inductor 7 after the voltage is rectified and smoothed and operates a push-pull inverter, a flyback phenomenon occurs. Then an electric current flows to a large-capacity flyback voltage storing capacitor 6 for reducing ripple from a flyback diode 8 through the auxiliary winding of the inductor 7 and, further, toward the power source 1 through a reverse-current preventing diode 5 so as to discharge the energy stored in the main winding of the inductor 7. The capacitor 6 is charged with this current only and the charging/discharging currents of the capacitor 6 do not work as a capacitor input type without receiving any direct influence from an AC input current from the power source 1. Consequently, the power factor of the AC input power is improved.