In air conditioner having a power-variable compressor, an inverter for varying the driving frequency of the compressor through V/f control, a controller for supplying a variable-frequency control instruction in accordance with the air conditioning load to the inverter, and an input voltage detector for detecting a source voltage input to the inverter, the controller includes reference means for beforehand storing V/f value data corresponding to plural voltage sections to which a predetermined range of the input voltage to the inverter is divided at a predetermined voltage interval, and selects data representing the ratio (V/f) of the output voltage (V) corresponding to the detected input voltage and the driving frequency (f) from the reference means to output the variable frequency control instruction to the inverter so that an air conditioning operation is carried out in accordance with the air conditioning load.