PURPOSE: To enable speedy suction and exhaustion of inert gas inside an oil tank by providing a suction and an exhaust valve to be closed by weights in a suction and exhaust air cylinder.

CONSTITUTION: A suction and exhaust air cylinder 2 is provided on a tank upper board 1 with an exhaust valve 9 to be closed by a weight 8 and a suction valve 13 to be closed by a weight 12. Besides, a lid body 16 with a weight 17 is provided outside the exhaust valve 9. When oil is injected into an oil tank filled with inert gas, the exhaust valve 9 and the lid body 16 are opened by internal pressure to exhaust the inert gas quickly. When the internal pressure is varied by some cause, the suction and the exhaust valve are operated to maintain a fixed internal pressure.