PURPOSE: To conduct the operation of an apparatus at the optimal operating point by obtaining a reference power factor, when a primary current is minimized, and by correcting a voltage so that a power factor coincides with the reference power factor.

CONSTITUTION: A commercial power 1 is supplied to the motor of a compressor 4 via a converter 2 and inverter 3. A power factor is detected by a power-factor detecting means 11, and when there is a stored value in a reference power-factor storage means 13, a power-factor comparing means 10 controls the inverter 3 via voltage correction means 9 so that a reference power-factor stored value and a power factor detected value coincide with each other. When there is no stored value in the reference power-factor storage means 13, a command voltage to the inverter 3 is changed by a voltage sweep means 7, and a power factor value at the time of minimizing an electric current detected by a primary current detecting means 5 is stored as a reference power factor in the reference power-factor storage means 13.