To enable an important load to be alive even when using a single-feed type generator.

The power from a commercial power source 24 is fed to the important load 23 through an inverter 26. As the inverter 26 comprises a parallel-operation mode, also the single-feed type generator 25 can feed power connected in parallel with the important load 23. When the generator 25 malfunctions, a detecting unit 30 detects the malfunction, and a control unit 31 cuts off a switch 22 and shifts the inverter 26 to a constant-voltage constant- frequency operation mode. When a service interruption occurs at the side of the commercial power source 24, a switch 21 is cut off, and only the power from the generator 25 is fed to the important load 23. Not only the important load 23 but also a non-important load is connected to a parallel line 28 and, when a single operation of the generator 25 is conducted, or when a single operation at the side of the commercial power source 24 is conducted, the only the important load is enabled to be alive by cutting off the non-important load.