PURPOSE: To eliminate the necessity of large rotary torque for opening/closing a valve, by shifting the center of a valve rod to the outer circumferential side of a valve body from the center of the valve body and differentiating the areas between the opposite sides of the valve rod.

CONSTITUTION: When a solenoid valve 21 is opened due to the change of the ambient condition, pressurized carbon dioxide flows from a gas cylinder 19 into a right cylinder chamber 31 so as to push a right piston 35 to the left. Consequently, a valve rod 13 is rotated in the direction of an arrow by a scotch yoke mechanism comprising a pin 45 and a scotch arm 41 to open a valve body 15. The center of the valve rod 13 is shifted from the center of the valve body 15 to the outer circumferential side of the valve body and the areas at the opposite sides of the center of the valve rod are differentiated. Furthermore, the center of the valve rod 13 is shifted from the center of the valve body 15 to the up/down stream direction of the flow path thus providing a double eccentric structure.