PURPOSE: To obtain constantly a smooth shutter motion by a method wherein two shutters to open and close an air supply port and an air discharge port respectively are driven to open and close the shutters at the same time by an associated movement of gears, and the shutters are arranged in a central part of the backside of a body.

CONSTITUTION: When a ventilation door is operated, a pulling string 16 is pulled to rotate a connection part 12c connected to the pulling string 16 around a bearing part 12a, so that a gear 12b provided outward the bearing part 12a may be rotated, and further, another gear 13b engaged with the gear 12b may be rotated in association around another bearing part 13a. By this, shutters 12 and 13 fixed to the bearings 12a and 13a are moved to the positions shown by dotted lines, to open an air supply port 4 and an air discharge port 5. At this time, the pulling string 16 is held at the position by an interlocking mechanism 15. When the operation is stopped, the pulling string 16 is operated to release the locking of the interlocking mechanism 15, make the gears 12b and 13b rotate together with the rotation of the connection part 12c and close the shutters 12 and 13.