Opposite ends of a first film-like damper with a plurality of air vents defined therein at a given position are wound respectively on first and second takeup shafts, and a drive wire is coupled to the first and second takeup shafts. A wire has an end directly coupled to the drive wire at substantially intermediate position between the first and second takeup shafts. The other end of the wire is connected to a manually operable knob. When the knob is manually turned, the wire directly displaces the drive wire in the direction indicated by the arrow A or B, moving the film-like damper.