A width of a groove of a cathode holder and a thickness of a cathode conductor are determined so that a dimension which is obtained by subtracting the thickness from the width is equal to a gap length which has a predetermined length, and which is between a filament and a cathode. Then, the cathode holder is rearward pushed to cause a front end face of the groove 26 to butt against the cathode conductor, and the cathode conductor and filament conductors are coupled and fixed to each other via an electrically insulating material. And then, the filament is forward moved to butt against the cathode, and the filament is fixed to the filament conductors. After that, the cathode holder is forward pulled to cause a projection to butt against the cathode conductor, and the cathode holder is fixed to the cathode conductor.