PURPOSE: To suitably selectively use a solar cell and a commercial power source in response to a magnitude of a load by connecting the cell and the power source in series with a compressor in a room air conditioner to be driven by the cell as a power source of the compressor.

CONSTITUTION: AC power from a commercial power source E is input to a mixture bridge rectifier having a plurality of thyristors S1, S2 and diodes D1, D2 through a power factor improving reactor L, and a DC voltage Vd is formed by a smoothing capacitor C1. Three-phase AC voltages are formed by switching of an inverter 1 by utilizing the voltage Vd to control a rotating speed of a compressor motor 3. In this case, a solar cell Es is connected to the rectifier through a plurality of diodes D3, D4 for preventing a reverse flow from the power source E to the cell Es. Information of the voltage Vd is input by a controller 2, and the thyristors S1, S2 are controlled to be switched.