Abstract: A tubular glow discharge electrode is formed therein with a slot in which front end parts of support leads are inserted. The slot has a widened center part which defines an opening. The tube-like glow discharge tube is welded at its one end part to one of the support lead so as to be electrical conductive to the same, and is caulked at it's the other end part to the other one of the support leads through the intermediary of an insulator. A filament electrode (ark discharge electrode) carrying an electron-emissive substance is supported to the free end parts of the support leads within the tube-like glow discharge electrode which therefore surrounds the arc discharge electrode with a small gap defined between the glow discharge electrode and the filament electrode.