PURPOSE: To achieve the improvement of power factor and the prevention of a rush current with simple circuit constitution, in case that the power source of a half bridge type of switching regulator out of a rectifier is constituted.

CONSTITUTION: A half bridge inverter circuit consisting of first and second switching elements Q1 and Q2, capacitors C11 and C12, and a transformer 7 is connected between a pair of rectified output terminals 5 and 6 of a rectifier 1. A series circuit composed of a smoothing capacitor C1, a reactor L1, and a first diode D1 is connected in parallel with the first switching element Q1. A second diode D2 is connected in parallel with the capacitor C1 and the reactor L1. A third diode D3 is connected between the other end of the second switching element Q2 and the other end of the capacitor C1. The output voltage is controlled by turning on or turning off the first and second switching elements Q1 and Q2 for voltage control with PWM pulses, and also the current of the capacitor C1 is intermitted.