PURPOSE: To improve a cooling efficiency as a whole by uniformly cooling in a first heat exchanger, introducing refrigerant of gas and liquid mixture state outputted from the exchanger to a second heat exchanger, and cooling CO laser gas outputted from a discharge chamber.

CONSTITUTION: Laser gas to be introduced to a discharge chamber 4 is cooled by liquid nitrogen introduced from a liquid nitrogen tank 11 through a tube 12 in a first heat exchanger 6. The liquid nitrogen is so supplied that part is retained as liquid even at the outlet of the exchanger 6, the gas is uniformly cooled, and introduced to the chamber 4 in a state that no temperature distribution exists in the lateral direction of a duct 3. Accordingly, a stable laser light can be obtained in a resonator 5. Mixed phase nitrogen outputted from the exchanger 6 is introduced to a second heat exchanger 7 through a refrigerant tube 8, and raised at a temperature by glow discharging. Laser gas outputted from the chamber 4 is cooled by the mixed state nitrogen, the nitrogen all become gas at the outlet of the exchanger 7, the liquid nitrogen is used to cool laser gas without loss, efficiently.