PURPOSE: To facilitate stable control of a laser output over a wide range by a method wherein a plurality of pairs of electrodes are provided on a laser tube and a discharge is induced between an optional pair or optional pairs of the electrodes.

CONSTITUTION: Mixed gas of CO2, N2H and the like is enclosed in an insulating cylinder 11. A total reflective mirror 12 is provided at the one end of the insulating cylinder 11 and an output mirror 13 is provided at the other end. An optical resonator is composed of both the mirrors. A plurality of pairs of electrodes 14a and 15a∼14c and 15c are provided on the outer circumference of the insulating cylinder 11 facing each other so as to hold the insulating cylinder 11 between them, they are connected to an external radio frequency electric source 16 through switches 17 and 18 respectively in parallel. If the main electrodes 21 and 22 of the switches 17 and 18 are brought into contact with all contacts 19a∼19c and 20a∼20c respectively, a discharge is induced over a whole discharge region and the maximum laser output can be obtained. If the main electrodes 21 and 22 of the switches 17 and 18 are brought into contact with the one respective contacts 19a and 20a, a discharge is induced between a pair of the electrodes 14a and 15a only and a laser output corresponding to the length of the discharge region can be obtained.