PURPOSE: To provide an actuator for butterfly valves having simple constitution so that it can be economically manufactured and strong and reliable at operation.

CONSTITUTION: A tube member 4 is uprightly provided on the center of the bottom cover 2 of an upper cylinder housing 6. A rod 8 supports a piston 10, it continues to a pin part 12 under the piston, and the pin part is connected with a tubular sleeve 16. The sleeve comprises opposed axial notches 20 for receiving a fixing lateral pin 18 on the tube member 4, and a helical track 26 cam-engaged with a pin 24 projecting from a rotary output shaft 22 inside the tube member 4. A compression spring 32 is inserted between the bottom cover and the underside of the piston 10. The piston is downward pressed by compressed air to bring down the sleeve, and the rotary output shaft is rotated with the helical track and the pin 24.