Ahead portion (150) is provided at one end of an outer circumferential wall (101) capable of elastic compression deformation. The outer circumferential wall includes multiple compression units (110, 120) that undergo deformation due to compression force in a central axis (100a) direction. Letting N be an integer of 2 or more, the compression units each have at least either N thick portions (113) or N thin portions (123) formed at equiangular intervals with respect to the central axis such that the thickness periodically changes in the circumferential direction. The phase with respect to the central axis of the periodic change in thickness is shifted between two compression units that are neighboring in the central axis direction.
