PURPOSE: To inhibit adhesion of, floating foreign matters in a tube, to an output mirror.

CONSTITUTION: An anode 3, a thin discharge tube 2, and a cathode 1 in a vacuum enclosure 4 are arranged in cascade wherein the anode side space is communicated with the inner space of the vacuum enclosure through a gas return path 15. Furthermore, an output mirror 8 is fixed to the anode side end part while a high reflection mirror 7 is fixed to the cathode side end part and the anode side end 16 of the gas return path is extended like a nozzle to the vicinity of the optical path.