PURPOSE: To make a discharge in a glass tube uniform without again forming an opening having a mode selecting function ]by providing a metallic part for fixedly supporting the tube, and disposing its opening for selecting a mode at the metallic part.

CONSTITUTION: A glass tube 9 held by metal electrodes 12 is supported by a support 10 and a retainer 14 through an O-ring 15, and secured together with a manifold 11. Such a support 10 can be mounted over entire both ends of the tube, its inner diameter is different depending on the oscillation mode of a necessary laser light, and selected in response to a position in a resonator. An RF discharge is generated in the tube held by the electrodes to excite laser gas. In this case, partial discharge from a protrusion as a center is not generated in the tube, and a uniform discharge can be formed over the entire region held by the electrodes.