To enhance safety by connecting a first diode in the same direction as the DC output terminal of a full-wave rectifier, so that a current flows from an inductor through a second diode to a load circuit when a switching element is turned off, thereby preventing an undue input power from being fed, with respect to the output power.

The mode is switched when the sum (Vc1+Vz) of the voltage Vc1 of a capacitor C1 and a load voltage Vz varies between the full-wave rectified output voltage Vin of an AC power supply P and the charged voltage VEC of a smoothing capacitor EC. When a switching element Q is turned off and Vin<Vc1+Vz=VEC, a current flows through a path of an inductor LM, the capacitor C1, a diode D1, the smoothing capacitor EC and a diode D2 and a current also flows through a path of the inductor LM, a load circuit Z and the diode D2. According to the arrangement, an undue power is prevented from being fed with respect to the output power at the time of increasing the load, circuit current, and the safety is enhanced.