PURPOSE: To minimize a stain caused by moisture and dust coming into a switchboard, so as no prevent deterioration of insulation performance of the switchboard and a trouble therein by closing a ventilating port at the time of low internal temperature and opening the ventilating port at the time of high internal temperature in the switchboard or the like.

CONSTITUTION: One side end of a doglegged shape memory alloy 2 is mounted to a casing 1 of a switchboard provided with a ventilating port 3, and a ventilating plate 4 is mounted to the opposite end of the shape memory alloy 2. When internal temperature of the switchboard is at a fixed value or more, the shape memory alloy 2 is formed in doglegged shape to open the ventilating plate 4, but when the internal temperature of the switchboard is decreased to the fixed value or less, the alloy is deformed in almost linear shape accordingly to close the ventilating port 3 by moving also the ventilating plate 4 to a position 4A. In this way, the ventilating port is closed, at the time of low internal temperature of the switchboard, and opened first, at the time of high internal temperature, to perform ventilation, so that deterioration of insulating performance of the switchboard can be prevented by decreasing an internal stain by the amount of the ventilation.