To prevent swinging of a wind mill blade of a horizontal-shaft wind mill by supporting a part around a jointed portion between a driving body near a blade and a driven body such as a generator on a tower, setting the rotational direction of a driven body in the opposite direction to that of the driven body, setting each angular momentum so as to be equal to each other, and compensating for a gyro moment generated in a head-to-head manner from both driving/driven sides for the supporting portion when the external disturbance of disturbing wind power due to a wind direction fluctuation, a tower, a building or the like.

Before a speed change to opposite rotation is transmitted to the driven body 2a such as an armature 2b of the generator 2 through a transmission 3 by the driving body 1a including the horizontal-axis wind mill blade 1, the angular momentums J1ω1 and J2ω2 are previously to be equal to each other for an angular speed ω1 and a moment of inertia J1 of the driving body 1a and an angular speed ω2 and a moment of inertia J2 of the driven body 2a, and a portion beneath the transmission is supported on the tower 5 by a supporting shaft 4.