PURPOSE: To prevent overheat by improving the cooling of a cathode by a method wherein the cathode is inclined to the direction of upper stream of gas.

CONSTITUTION: An anode 1 and the cathode 2a projected to the side of the anode 1 are arranged in opposition by holding a laser beam therebetween, and gas is made to flow between the both poles. At the time, a projected discharge end part 9a inclined to the direction of the upper stream of the gas is formed on the cathode 2a. Then, the gas whose temperature increases by the discharge flows to the root side of the end part 9a, but the cathode is not overheated because of being cooled by heat transfer to the mounting part of the electrode 2a.