PURPOSE: To simplify the structure and to reduce the price, by providing a discharge tube in a cavity oscillator having a cooling device, generating microwave discharge in the discharge tube, exciting a laser medium gas, and cooling heat yielded by the microwave discharge by the cooling device.

CONSTITUTION: A magnetron 4 outputs microwave power based on the output from a power source 13. The power is guided into a cavity oscillator 11 through a waveguide 15 and resonance is generated. Then an electric field is enhanced and microwave discharge 17 is generated in the discharge tube 1. The microwave power is absorbed by the microwave discharge 17. A laser medium gas 2, which is circulated in the discharge tube 1 through a heat exchanger 3 by a blower 14, is excited. Laser oscillation is induced between a total reflection mirror 8 and a partial transmitting mirror 9. Laser light 10 is taken out of the partial transmitting mirror 9. The temperature of the inside of the cavity oscillator 11, especially that of the discharge tube 1, becomes high due to the heating by the microwave discharge 17. Therefore a gas 19 is sent to the cavity oscillator 11 from a cooling device 18, and cooling is performed. The gas 19 is exhausted through an exhausting port 20.