PURPOSE: To withstand high voltage even when an enclosure is made shorter and more compact by providing a cylindrical shield with a gap held between the enclosing wall and a cathode.

CONSTITUTION: A metallic cylindrical shield 10 which is stretched toward a cathode 2 is mounted on an anode terminal 5 with a proper interval against an enclosure 1 or the cathode. In this case, the gap from the cylindrical shield 10 to enclosure 1 need not be cared unless mutual contact is suspected, but the gap to the cathode 2 must be large enough to withstand the high voltage applied between the cathode 2 and an anode 7. The tip area 10a of the cylindrical shield 10 must at least protrude from the top level of an anode cylinder 4 toward the cathode 2.