PURPOSE: To prevent dusts from mixing in equipment to reduce the noise of equipment and to facilitate the heat control in the equipment by forming an opening in a variable area, and so controlling the area of the opening as to hold its inner temperature in a predetermined range.

CONSTITUTION: The temperature for increasing the area of an opening in office equipment 11 is set to T°C. When this temperature arrives at the vicinity of the T°C, a compression coil spring 4 is pressed to be returned by the recovery force of a shape memory alloy coil 5, and a member 3 with a rack is moved leftward. Simultaneously, a pinion 2 is rotated clockwise, and a shielding plate 1 with a slit is moved rightward. That is, the relative position between the vent hole 12 of a sheath cover 6 and the slit 8 of the plate 1 is moved thereby to increase the opening area for ventilation of the atmospheric aid. If the inner temperature of the equipment drops, the member 3 is again pressed rightward by means of the compression force of the spring 4, the pinion 2 is rotated counterclockwise correspondingly to return the plate 1 leftward. Accordingly, the opening is closed.