PURPOSE:To miniaturize a discharge chamber, and to scale down the whole device and improve efficiency by constituting the discharge chamber of a ceramic member. CONSTITUTION:The sidewall 1a and upper wall 1b of a discharge chamber 1 are composed of a ceramic member such as machinable ceramics. Accordingly, no trouble is generated even when the thickness of the discharge chamber is thinned in comparison with a metallic member consisting of aluminum, etc. and a synthetic resin group member made up of Teflon, etc., thus miniaturizing and lightening the whole device. There is no possibility in which the member is corroded by a reactive gas.