A sealant 34 filled into a cap 30A is caused to overflow through both an opening in a first end portion 30a of the cap 30A and a through-hole 40 of the cap 30A when the cap 30A is pressed against a fastener member 25. With regard to the cap 30A, a ratio between inside diameter D1 of the opening on the side of the first end portion 30a and hole diameter D2 of the through-hole 40 in the second end portion 30b is set as follows: 0.10≦D2/D1≦0.27.