An air conditioner is disclosed. The air conditioner includes an economiser heat exchanger (150) for evaporating refrigerant, a compressor (110,112) for compressing refrigerant, an injection flow channel (158a,158b) through which the evaporated refrigerant flows into the compressor, an injection valve (159a,159b) for opening and closing the injection flow channel, an oil separator (120,122) for separating oil from refrigerant discharged from the compressor, and an oil injection line (122b), which communicates at one end thereof with the oil separator and communicates at the other end thereof with the injection flow channel so as to guide the oil separated by the oil separator toward the injection flow channel, wherein the oil separated by the oil separator selectively flows into the compressor through the oil injection line and the injection flow channel in accordance with a mode of operation of the air conditioner.