PURPOSE: To avoid the temperature rise of discharge electrodes by a method wherein plate-shape radiation members which are brought into contact with a plurality of rod-shape discharge electrodes are provided along the direction of a gas flow.

CONSTITUTION: A plurality of rod-shape discharge electrodes 5 are attached to the upper inner surface of the walls 1 of a discharge chamber along the direction of a gas flow G by insulating supporters 3 and plate-shape radiation members 7 are provided on the left side of the electrodes 5 in the upstream part of the gas flow G. A discharge electrode 9 which forms an electrode part with a group of the electrodes 5 is attached to the lower inner surface of the walls 1 by a supporter 11 so as to face the electrodes 5. Feeders 13 and 15 are connected to the electrodes 5 and 9. As the members 7 are efficiently cooled by the low temperature gas flow G, the electrodes 5 are sufficiently cooled. As the electrode 9 is plane-shape and has a large area, it is cooled sufficiently by the gas flow G and no radiation member is required. With this constitution, the temperature rise of the discharge electrodes can be avoided, so that a large current can be applied while long life is maintained.