A discharge electrode E in an electrode chamber C comprises a plurality of electrode members 8, 9. The electrode members 8, 9 are disposed facing each other by having a supporting member 4 therebetween, a gap is formed between the facing portions of the electrode members 8, 9, and by having the gap as a gas passageway 15, the gas passageway is opened in the leading end of the discharge electrode. A replacement gas having been supplied from a manifold pipe 3 is supplied to the gas passageway 15 via an orifice.