PURPOSE: To reduce the size of a power system by using a small-size and small- capacity circuit component in a rush current preventing circuit of the power system which has a capacitor for smoothing the output.

CONSTITUTION: A switch SW1 and a switch SW2 are inserted in series with a power line between a rectifier bridge 2 which full-wave rectifies ac inputted from an ac supply 1 and a capacitor for smoothing the output CO. Between the switch SW1 and the switch SW2, a series circuit of a resistor for limiting current R1 and a capacitor for auxiliary charging C1 is connected in parallel with the capacitor for smoothing CO. When the power is turned on, the on and off of the switches SW1 and SW2 is controlled with a specified timing by means of a controlling section 4 to charge and discharge the capacitor C1 by installments and then charge the capacitor for smoothing CO gradually.