Abstract: A sealing member 11 made of an elastic material secured to the outer peripheral edge of a door plate 10 is provided with support portions 11a and 11b that extend in a direction to intersect the door plate 10, and the support portions 11a and 11b are directly supported by the outer peripheral edge of the door plate 10. Sealing portions 11c and 11d pressed onto a sealing surface 12a of an air passage casing 12 are so formed as to be directed toward the central portion of the door from the support portions 11a and 11b, and the sealing member 11 is folded, in cross-section, from the outer peripheral edge of the door plate 10 toward the central portion of the door due to the support portions 11a and 11b and the sealing portions 11c and 11d.