PURPOSE: To allow an air conditioner to be operated in low capacity and to perform finer control, by setting the lowest frequency with which the air conditioner can be operated lower than the starting frequency in starting it and by preserving V-F characteristic for less voltage sag against the drop of frequency in low frequency range.

CONSTITUTION: An air conditioner is equipped with a compressor 1, a four-way valve 2, an outdoor heat exchanger 3, an expansion valve 4 and an indoor heat exchanger 5, among which the compressor 1 is driven with variable speed by an inverter device 6. The V-F output characteristic of the inverter device 6 makes the slope of a V-F characteristic curve different with the startlng frequency Fs (e.g. 28Hz) at its peak, i.e. in the low frequency range below the stating frequency Fs the voltage sag against the attenuation of frequency is reduced. The reduction of rotating torque of the compressor 1 against the attenuation of frequency in low frequency range can thereby be controlled.