PURPOSE: To conduct soft start slowly raising an output by simple circuit constitution, cost of which is reduced.

CONSTITUTION: When DC voltage is applied to a resistor R4 and the primary winding L1 of a transformer 7 from a DC power supply 1 at first, DC currents flow through the resistor R4, a Zener diode 10 and a capacitor C4, a transistor 5 is maintained at the Zener potential of the Zener diode 10, the base potential of the transistor 5 is elevated with the charging of the capacitor C4 and the conductive resistance of the transistor 5 is lowered gradually, the transistor 5 is brought to an on state slowly, and switching is started. Switching is started, and negative potential fed from a diode 9 is applied to the anode side of a Zener diode 11, thus preventing the effect of the series circuit of the Zener diodes 10, 11 on the switching operation of the transistor 5.