PURPOSE: To prevent the over-rotation of a windmill by constituting it so that a link mechanism is disengaged when windmill blades are rotated at a predetermined speed or more, the blade angle is changed to an angle not generating a rotating force, and it is not returned unless an external force is applied.

CONSTITUTION: When an operation shaft 16 is moved backward by an operation machine 17, an auxiliary link bar 22 expands a link bar 18 outward. This condition is the same as that when a regulating weight 27 is operated by means of a centrifugal force, and a windmill is stopped rotating. To start the windmill operation, the opration shaft 16 is moved forward by means of the remotely controlled operation machine 17. Thereby, the auxiliary link bar 22 is withdrawn inward and the link bar 18 is also withdrawn inward, and a blade shaft 4 is changed to the operation angle of a twist blade 4 through an operation shaft 8, a connecting rod 10, and a blade shaft arm 6 to obtain a normal operational condition.