PURPOSE: To enable the increase in output, prolongation in lifetime, and miniaturization of a discharge oscillator by a method wherein the cathode using a hollow cathode is flattened in cross section.

CONSTITUTION: A cathode 18 is composed of the outer electrode 11 of cylindrical form opened on the side opposed to anodes 9 and the inner electrode 12 provided inside this outer electrode 11 by alienation from this electrode 11 and is formed into a member having the appearance of flat cross-section in the direction not to block flows of the excitation gas 8. This enables the generation of discharge between a slit 19 and the anodes 9 on the start of discharge. When a plasma 20 is thus generated in the slit 19, the plasma 20 creeps to the discharge space 13, and a current with a density several times that of a normal glow discharge current is led out of the hollow cathode 18 by the exhibition of hollow cathode effect, accordingly, the plasma 20 of high current-density is generated between the anode 9 and the hollow cathode 18. In other words, the laser output increases because of the increase in power density.