A power factor improving system for a power supply system, comprising an AC power supply, a rectifier circuit connected to the AC power supply, a smoothing circuit for smoothing the output current of the rectifier circuit and supplying DC power to the load, an inductance element inserted in series between the power supply and the rectifier circuit, switching means including a switching element and a rectifier element inserted between the AC input terminal of the rectifier circuit and one of the DC output lines thereof, and a control circuit for subjecting the switching element of the switching means to on-off control at a frequency higher than the frequency of the AC power supply.