PURPOSE: To easily and accurately make relative positioning between a focussing electrode and a cathode filament.

CONSTITUTION: A filament positioning jig 13 which is provided with a projection 15, 16 having a step part 17, 18 corresponding to a reverse V-shaped plane corresponding to the V-shaped plane 2 of a focussing electrode 1 and a focussing groove protruded from the reverse V-shaped plane 14 is used. The V-shaped plane of the filament positioning jig is arranged to abut against the V-shaped plane of the focussing electrode and at the same time the projection is fitted into the focussing groove, and cathode supporting bars 7 to 10 in which the leg part of a cathode filament 6 is firmly fastened on the end part in advance, are respectively inserted into sleeves 11, 12. The coil part of the cathode filament is placed on the step part of the projection and the cathode supporting bars are firmly fastened to sleeves as they stand.