PURPOSE: To ensure stable oscillation at high outputs and for a long period or time, by controlling a flow velocity or flow rate of laser gas in accordance with an output of the output detector so as to keep the pressure, temperature, distributing condition, etc. at their optimal values for laser oscillation.

CONSTITUTION: CO2, N2 and H gases are mixed in a predetermined composition and then supplied into a straight pipe 1a. On the other hand, laser gas adsorbed from the straight pipe 1a is cooled and oxidized by a heat exchanger 29 and a filter 34 to be returned to the straight pipe 1a. The flow rates of these gases are detected, respectively, by flowmeters 26 and 28, and the detected values are inputted to a control unit 39. The laser ocillation output oscillated from the laser tube 1 is detected by an output detector 38, and the detected value is stored in the control unit 39. The control unit 39 controls a gas flow control valve 25 and a fan 35 for draw-in gas according to a predetermined program, so as to keep the pressure, temperature, condition of distribution and composition of the laser gas in the straight pipe 1 at constant values suitable for laser oscillation.