PURPOSE: To prevent projecting phenomenon caused through utilization of pneumatic pressure, by adding a mechanism for producing frictional resistance continuously onto the rotary shaft of a rotary actuator.

CONSTITUTION: Rotary shafts 4, 5 of a butterfly valve 1 and a rotary actuator 3 are coupled eccentrically each other through a four joint link 6. A frictional ring member 12 is pushed by a stopper board 13 through bolts 14, 15 in the axial direction of a rotary shaft 5 at one end thereof. Such material as having quite small difference between dynamic friction coefficient and static friction coefficient thereof is selected for the friction member 12 in order to suppress projecting phenomenon due to pneumatic operation and improve the accuracy of valve opening.