A seal member is a member including a lip, a seal base, a first extension section smaller in thickness than the seal base, and a first locking section, all of which are formed to be continuous in the cross-sectional direction. A retainer plate includes a base accommodating concave section for accommodating the seal base, and a first engagement concave section for accommodating the first locking section, and controls the first locking section to prevent the seal member from falling off. The first locking section is accommodated in the first engagement concave section with gaps left in the thickness and cross-sectional directions.