PROBLEM TO BE SOLVED: To stabilize a positional relation in a radial direction between an outer-covering cylindrical part and a head main body by restricting oblique pushing-down of a push-down head.SOLUTION: A fitted cap 11 comprises an annular top wall part arranged on a mouth part and a cylindrical guiding part 17 arranged in a protruding state in an inner peripheral edge of the top wall part. A push-down head 15 comprises: a head main body 41 that has a top wall part 43 arranged above a stem 12 and a fitted cylindrical part 44 that extends downward from the top wall part 43 and is fitted to the stem 12; and an outer-covering cylindrical part 42 that is provided below the top wall part 43 and surrounds the fitted cylindrical part 44 and the cylindrical guiding part 17 from outside in a radial direction. The head main body 41 and the outer-covering cylindrical part 42 are formed independently and assembled to each other. On an inner peripheral surface of the outer-covering cylindrical part 42 is arranged in a protruding state a guide rib 53a extending in a vertical direction.
