An optional unit (30) including a high pressure side compressor (31) and a gas-liquid separator (33) is provided between an outdoor unit (20) including a low pressure side compressor (21) and an indoor unit (40). During a defrosting operation, refrigerant discharged from the low pressure side compressor (21) defrosts an outdoor heat exchanger (21) while refrigerant in the gas-liquid separator (33) is sucked into the high pressure side compressor (31) and is then discharged to the suction side of the low pressure side compressor (21). The optional unit (30) includes an injection pipe (36) for returning part of the refrigerant discharged from the high pressure side compressor (31) to the suction side of the high pressure side compressor (31) in the defrosting operation. Hence, the refrigerant flowing into the high pressure side compressor (31) from the gas-liquid separator (33) is gasified by the high-temperature discharged refrigerant.