A cathode holder of a tubular shape is inserted into an opening for a cathode of a plasma generating chamber, the cathode holder positioned such that a surface thereof opposes or surrounds a side surface of a cathode. The cathode is held in the cathode holder so that a front surface of the cathode will be positioned on the same plane as, outward from, or inward from the inner wall surface. In the cathode holder is provided a tubular first heat shield surrounding the cathode with a space provided between the first heat shield and the cathode, a surface of the first heat shield positioned to oppose or surround the side surface of the cathode. At a rear end of the cathode is provided a filament. The gap between the cathode holder and the plasma generating chamber is filled with an electrical insulating material.