To provide an air conditioner the cooling power of which can be improved by lowering the enthalpy of a refrigerant before the refrigerant flows in an evaporator.

This air conditioner is provided with a compressor 1, a gas cooler 2 which condenses a CO2 refrigerant compressed by means of the compressor 1, and a pressure reducing means which reduces the pressure of the CO2 refrigerant condensed in the cooler 2. The air conditioner is also provided with the evaporator 4 which evaporates the CO2 refrigerant reduced in pressure by means of the pressure reducing means. In this air conditioner, an expander 3 which aggressively reduces the pressure of the CO2 refrigerant by expanding the refrigerant is used as the pressure reducing means.