An object of the present invention is to increase a laser output and improve the insulating performance. The device according to the present invention comprises a pair of main discharge electrodes, a dielectric and an auxiliary electrode, the dielectric being formed to be cylindrical and having the side opposed to the main discharge electrodes formed into a flat plate-like portion, the auxiliary electrode being embraced within the dielectric leaving a space and being opposed to one of the main dishcarge electrode with the flat plate-like portion sandwiched therebetween.
