DAMPER

PROBLEM TO BE SOLVED: To provide a damper that can prevent the occurrence of an excessive large air quantity in a natural ventilation system. SOLUTION: This excessive air quantity preventing damper 1 is formed of a frame member 2 connected to a duct (a) and a main blade 3, an auxiliary blade 5, etc., which can rotate in the duct (a). When a favorable wind blows, the auxiliary blade 5 rotates in the direction in which the quantity of the air passing through the duct (a) increases and, at the same time, the main blade 3 receives a dynamic lift and rotates in the direction in which the quantity of the air passing through the duct (a) decreases as the quantity of the air flowing in the duct (a) increases. When the quantity of the air flowing in the duct (a) becomes excessively large, first and second wind receiving sections 3a and 3b formed on the main blade 3 receive air pressures and the main blade 3 rotates to a position where the blade 3 makes the quantity of the air passing through the duct (a) almost zero. Consequently, the occurrence of the excessively large air quantity in the duct (a) is prevented.