PURPOSE: To improve accuracy of assembly, by fixing a cathode eyelet supporter and a cathode insulator stone to the retainer into an unit assembly and fitting this assembly into a cup shape space in the control electrode and fix it there.

CONSTITUTION: A cathode insulator 16 formed by sintering and molding alumia ceramics with high purity is inserted into a cup-shape space in retainer 18, further a cathode support 17 made of hermetic material and provided with a cathode eyelet support 6 is also inserted in this space and fixed, thus a retainer assembly 19 is built up, subsequently, cathode 5 is welded and fixed to the cathode eyelet support 7 so that the interval between the electron radiation layer 15 and control electrode 1 may be set to the prescribed value in reference to the surface 16a on which cathode insulator stone 16 touches the internal surface of the control electrode 1. And then, the retainer assembly 19 to which cathode 5 is welded is inserted the cup shape space in electrode 1 and the surface 16a is arranged to touch the internal surface of electrode 1 and welded to be fixed there. The accuracy of assembly can be improved through the above constitution.