A cover-side peripheral wall includes an inner wall, which serves as a first wall, and a bent wall. The inner wall is located between an inner wall and an outer wall of a body-side double-wall. The bent wall extends inward and downward from an inner surface of the inner wall. The bent wall has a proximal end that is located upward from an upper end of the inner wall of a body-side peripheral wall. The bent wall further includes a portion between a distal end and the proximal end that is located inward and downward from the upper end of the inner wall of the body-side peripheral wall.