| 摘要 |
In the present invention, a cap (50) has recesses (53a), low strength portions (55), and high strength portions (57). The high strength portions (57) are disposed at least in portions excluding the recessed portions (53a) in a direction about the axis of the cap (50). The high strength portions (57) are disposed in a line connecting a tip (53) and a base end (51) in the axial direction of the cap (50). |
