When a heating mode is set in a refrigerant cycle system, air is heated in a condenser (12) by condensing high-pressure gas refrigerant, a part of high-pressure refrigerant from the condenser is decompressed in a pressure reducing unit (26) to a middle pressure, and the other part of high-pressure refrigerant from the condenser is heat-exchanged with the middle-pressure refrigerant having passed through the pressure reducing unit in a refrigerant-refrigerant heat exchanger (23). Therefore, middle-pressure refrigerant having passed through the pressure reducing unit is evaporated in the refrigerant-refrigerant heat exchanger and the evaporated middle-pressure refrigerant is introduced into a gas injection port of a compressor (22). Thus, in the refrigerant cycle system, heating capacity can be improved due to the gas refrigerant injection into the compressor.