A cap member includes a cap portion and a support member. The cap portion has a bottom surface portion and a side surface portion. The support member supports the bottom surface portion of the cap member. The bottom surface portion is provided with a groove portion defined by a first rib projecting from an upper surface of the bottom surface portion. A lid member which covers the groove portion is provided on, and in close contact with. an upper end of the first rib. The groove portion and the lid member define an air passage through which air passes. One end portion of the air passage communicates with an inner space of the cap member, and the other end portion of the air passage communicates with an outside of the cap portion.