PURPOSE: To execute the reduction of the number of component parts or the like, by initializing a time constant circuit for setting the delay time of a rush- current-proof switching means according to voltage drop at a specified point, after the interruption of a power source.

CONSTITUTION: So far as a power-supply unit is concerned, the input of direct current obtaining an AC power source 1 via a switch 2, a bridge diode 3, and an input smoothing capacitor 4, is provided to the primary winding of a transforming transformer 6, and by keeping a switching element 8 intermittently with driving pulse from a driving circuit 10 including a stabilization controlling circuit, AC output is obtained on the secondary side of the transformer 6, and current is fed to a load 7, The transformer 6 is provided with another secondary winding to compose an auxiliary power source 9, and current is fed to the driving circuit 10. By the output of the auxiliary power source 9 with the time constant circuit of a resistor 16 and a capacitor 17, a thyristor 12 is driven, and the rush current of the smoothing capacitor 4 is reduced. By a circuit 23, a sufficient delay time is guaranteed even in case that an interruption period in the making period of the power source is short, and the working of the driving circuit is stopped.