When a retainer unit is fastened to a body, the rotation of a pin member around its longitudinal axis pushes out and widens the body mounting leg portion of the retainer unit, and the retainer unit is drawn into the body. As a result, the elastic flange of the leg is bent, the insertion hole in the body is sealed by the elastic flange of the leg, and infiltration of water from the outside is prevented. By drawing the retainer unit into the body, the pin member is also drawn into the body a slight distance (dimension α). As a result, the elastic flange of the pin member is bent, the pin insertion hole is sealed by the elastic flange of the pin member, and infiltration of water from the outside is prevented.