To reduce the cost by about 10%, as compared to those of prior art by reducing an apparatus capacity by a preliminary charger, which realizes a rush current suppressing means and a small installation space and small loss.

This DC power source has a DC-to-AC converter 5 connected to an AC power source Vs, via breaker 4 to DC-convert an AV power voltage Vs into supply a DC to a main power source 2 via a DC capacitor Ca. In this case, a preliminary charger 11 having small capacity DC power sources Vt, Tb, 13 and having a current-limiting resistor Rb is connected to a DC capacitor connecting point P via electromagnetic switches 12, 14 which are controlled by a voltage detector 7. Then before the AC power source Vs is connected, the capacitor Ca is charged preliminarily to a predetermined voltage by the charger 11. Then, the charger 11 is opened, the power source Vs is applied to suppress a rush current generation at the applying of the power.