A method for controlling an air cylinder such that air is supplied to and exhausted from pressure chambers of the air cylinder by air servo valves. Pressures in the pressure chambers are detected by pressure sensors, signals of the detected pressures are feedback to a controller, and the degrees of opening of the air servo valves are adjusted by corresponding PID adjusters of the controller on the basis of the respective differences between instruction values and detection values. The displacement of the rod of the air cylinder is detected by a displacement sensor, and a detection signal of the displacement is fed back to the controller so that a gain of the PID adjuster is always changed in accordance with the detection signal of the displacement.