The present invention aims to prevent, in a gas laser resonator, the deterioration in quality of discharge by reduction of the change of the pressure in a discharge chamber and the inflow of impurity gases, such as air, into the discharge chamber.A bracket 6 is attached to one end of a tube 1 interposing a gasket 13 only for sealing an opening of the discharge chamber 2 only and a gasket 14 for sealing both openings of the discharge chamber 2 and a buffer chamber 12. Also, a glass plate 8 and further a bracket 9 are attached to the other end of the tube 1 interposing a gasket 15 only for sealing the opening of the discharge chamber 2 and a gasket 16 for sealing both openings of the discharge chamber 2 and the buffer chamber 12. The pressure in the buffer chamber 12 is set lower than that of the discharge chamber 2 or set higher than the atmospheric pressure to decrease the inflow of the impurity gases to the discharge chamber 2.