When a control means turns on a pre-charging contactor, an AC current from an AC power source is converted into a DC current by a switching circuit to charge a smoothing capacitor. However, the smoothing capacitor cannot be charged completely due to the operation of a capacitor of an LC filter. Therefore, when the voltage of the smoothing capacitor detected by a voltage detecting means exceeds a predetermined value, the smoothing capacitor is further charged by bringing the switching circuit into a boosting operation. Thus, even if an LC relay required in the prior art for invalidating the capacitor of the LC filter is not provided, it is possible to completely charge the smoothing capacitor, thereby providing a decrease in the number of parts and an enhancement in reliability of the pre-charging circuit, and preventing a reduction in performance of the LC filter due to a loss in the LC relay.