To form a generator as a high frequency generator, so as to form it in small size, enhance reliability and reduce the cost, by a constitution providing a changeover switch in an input side of an uninterruptible power unit so as to selectively supply power from a commercial power source or the generator.

A power unit GS is constituted of a high frequency generator GH and a changeover switch SW, an uninterruptible power unit UPS is constituted by a rectifier circuit CH, inverter INV and a battery power unit B. A commercial power source input terminal Ts is connected to the uninterruptible power unit UPS through a terminal a of the changeover switch SW. An output terminal of the uninterruptible power unit UPS serves as an output terminal TO to a load, in a terminal b of the changeover switch SW, the high frequency generator GH is connected. In an emergency power unit, since a generator actuated at power interruption time is the high frequency generator, by a 50Hz or the like frequency condition of commercial power supply, a rotational speed is not limited to 3000rpm or the like, a frequency of power is rectified by the rectifier circuit CH in a device.