The damper plate

Peripheral ribs 34A, 34B of a damper plate 30 are raised from at least two sides along an extending direction of a rotation shaft 31 among four sides forming the peripheral portion of the damper plate 30 and have belt-like outer surfaces. When the damper plate 30 is positioned at a position to close a ventilation path 40, the outer surface of the upper peripheral rib 34A becomes parallel with an upper wall surface 13A of the retainer with a narrow clearance therebetween and the outer surface of the lower peripheral rib 34B becomes parallel with a lower wall surface 13B of the retainer with a narrow clearance therebetween.