To aim at improving welding strength through irradiation of laser, so that the whole fusing part of a cap is brought into contact with nickel.

By covering a welding face with the nickel, because the whole fusing part 21A of the cap 21 makes contact with the nickel, a sufficient welding strength is obtained, when the laser 22 is irradiated. For this reason, there is no need to irradiate with the strong laser 22, and the risk disappears of the electron emission agent 12 decomposing to generate gases and a hole opening in the cap fusing part 21A is eliminated. Further, it becomes possible to weld through irradiation the position of minimal requirement, and the decomposition of the whole electron emission agent 12 is avoided, and the welding time is reduced at the same time.