PURPOSE: To drive with a minimum force during starting and to obtain the greatest driving power during operation by applying a floating medium to a windmill.

CONSTITUTION: An arm 1 for a windmill is hollow. A fluid 2 contains a medium 3 consisting of grain-shaped solid. The medium 3 is placed on a base part 2. During starting of the windmill, the operation can be smoothly operated, because the medium 3 is placed on the base part 2. As the revolution increases, the medium 3 moves to a store case 4 located at the end of the arm 1, increasing the mass. As the revolution decreases, the medium 3 is naturally returned to the base part 2 from the store case 4. This construction makes it possible to increase inertia moment by moving the mass (medium), thereby obtaining a powerful operation.