PURPOSE: To prevent any warping of the filament by properly selecting the positions of the starting points of the filament according to the structures of end shields.

CONSTITUTION: When an upper end shield 2 works as an outside structure and a lower end shield 3 works as an inside structure, a filament 1 is installed in such a manner as to cause the upper-end-shield-side starting point 1a of the filament 1 to correspond to the lower-end-shield-side starting point 1b of the filament 1. In order to achieve such installation of the filament 1, either the distance (l) between the upper and the lower end shields or the effective length of the filament 1 itself is controlled. When the upper end shield 2 works as an inside structure and the lower end shield 3 works as an outside structure as well, the filament 1 is installed in such a manner as to cause the starting points 1a and 1b to correspond to each other. When both of the upper and the lower end shields 2 and 3 work as outside structure or inside structures, the filament 1 is installed in such a manner as to cause the starting points 1a and 1b to be at an angle of 180°. Owing to the above constitution, the central axis of the coil is not warped, the accuracy of the assembly is increased and any increased dark current is prevented.