To prevent a head part from being bitten into a holder or being damaged when it is rolling.

In this structure, an almost spherical head part 15c is formed at one end of an axial calotte 15. The head part 15c is rollingly fitted to a holder 20 and an almost concave head holding part 20k is formed by combination of two concave surfaces 20n, 20o having different positions A, B as their centers and a flat surface 20p located between the concave surfaces 20p, 20o.