An air conditioner comprises an outdoor unit and an indoor unit provided with an indoor heat exchanger and an indoor expansion device, wherein the outdoor unit comprises a compressor for compressing a refrigerant, an outdoor heat exchanger for heat-exchanging a refrigerant, a four-way valve adjacently arranged to the compressor for circulating a refrigerant discharged from the compressor according to a heating cycle or a cooling cycle, a refrigerant detouring path for detouring a refrigerant discharged from the outdoor heat exchanger to the compressor at the time of a defrosting operation, an outdoor expansion device installed in the middle of the detouring path for reducing a pressure of a refrigerant which flows in the refrigerant detouring path, and a heat exchanging device installed in the middle of the detouring path for heat-exchanging between a refrigerant introduced from the outdoor expansion device and a refrigerant discharged from the compressor.