PURPOSE: To accurately and conveniently assemble a lens abrration adjusting plate into a positive electrode by providing a supporting part to be welded to the bottom plate part of a sealed cup, and protruded lugs, to be inserted in the notches of the outer shell of a positive electrode, on an elliptic lens aberration adjusting plate.

CONSTITUTION: A lens aberration adjusting plate 9 has square-shape electron beam passing holes 9a, 9b, and 9c in-line arranged in an elliptic plate surface and also supporting parts 11a and 11b protruded from two long sides to be welded to the bottom plate part of a shield cup 10. Protruded lugs 12a and 12b formed on the short side of the adjusting plate 9 are inserted to the notches 13a and 13b of the short side of a cylindrical body 4 forming the outer shell of a positive electrode 3 to weld the bottom side part of the cup 10 and the end edge 4a of the cylindrical body 4. Consequently the adjusting plate 9 can be accurately assembled without position displacement into the cylindrical body 4.