The mounting tool 70 has a mounting insertion portion 71 to be inserted into the opening and a contact portion 73 to be brought into contact with at least a part of the non-function region portion 21 c. The attachment 70 further includes a positioning portion 77 for positioning the cap 50 and the attachment 70 with respect to the direction around the axis.装着具70は、開口部に挿入される装着挿入部71と、非機能領域部21cの少なくとも一部に当接する当接部73とを有する。装着具70は、軸周り方向に対してキャップ50と装着具70とを位置決めする位置決め部77をさらに有する。