PURPOSE: To reduce the dispersion in the degree of heater insertion, by providing the cathode sleeve contact part to normalize the degree of heater insertion to a part of the heater tab.

CONSTITUTION: When a heater constituent is inserted from the opening of the cathode sleeve 9, the comb tooth 12c1' and 12c2' for the upper planes are in contact with the opening end of the heater sleeve 9. After confirming the contact, the heater connector 10 and the comb tooth 12c1 and 12c2 of the heater tab 12 are welded by keeping the condition. Next, the linking part 12a of the heater tab 12 and the comb tooth 12c1 and 12c2 are cut off along the line B-B'. With this method, with very simple method that the contact of the cathode sleeve part to the cathode sleeve is confirmad, the heater can be assembled with good accuracy.