A base portion has a separation wall, and an air hole is defined in the separation wall. A case portion defines a first space with the separation wall of the base portion. A covering portion defines a second space with the separation wall of the base portion. A waterproof and breathable sheet covers the air hole. A water guiding portion is arranged adjacent to a surface of the sheet not to contact the surface of the sheet. The water guiding portion is located between the covering portion and the sheet.