PURPOSE: To stop electrons from flowing into an air gap from plasma so as to prevent electrical discharge from occurring by a method wherein insulator is filled into the air gap around an electrode to set it in width so as to make an electrostatic capacitance between the electrode and a dark space shield equal to that in a vacuum.

CONSTITUTION: An lower electrode 3 serving as a cathode is provided to the peripheral edge of an opening provided to the bottom wall of a chamber 1 through the intermediary of an insulating spacer 2. A dark space shield 12 is arranged separate from the peripheral end of the lower electrode 3 by a prescribed space. An air gap is formed between the dark space shield 12 and the lower electrode 3, and the air gap is so set in width as to make the capacitance of a cylindrical capacitor where the lower electrode 3 and the dark space shield 12 serve as electrodes equal to that in a vacuum. The air gap is compactly filled with a cylindrical insulator 13. By this setup, electrons are restrained from flowing into an air gap from plasma, so that an electric discharge is prevented from occurring.