PURPOSE: To prevent the generation of the diode oscillation and to obtain high withstanding voltage characteristics by constructing the lower portion of the cathode supporting cylinder of an electron gun smoothly tapered or curved and larger in diameter than the upper portion of which, and the lower portion of the insulating cylinder smaller in diameter than the upper portion of which.

CONSTITUTION: In a structure in which the outer surface of the cathode supporting cylinder 21 and the inner surface of the insulating cylinder 26 are faced to each other coaxially, the lower portion 21b of the cylinder 21 is made smoothly tapered or curved and larger in diameter than the upper portion 21a of which, and the lower portion 26b of the cylinder 26 connected to the stem portion is made smoothly tapered or curved and smaller in diameter than the upper portion 26a of which connected to the anode electrode 13 through metallic envelope 25 or the like. As the result, DC equipotential lines in the vicinity of the ceramic insulating cylinder 26 are generated as shown with broken lines E, so that the lines of electric force can be led out smoothly from the vacuum space to the outside of the tube with no local concentration of the energy. Thereby the generation of the diode oscillation can be made difficult and the withstanding voltage characteristics between the cathode and the anode can be improved to obtain high reliable performance.