To obtain rotational driving force for outputting the specified amount of energy by inputting and loading buoyancy balls to be raised in a water tank by buoyancy into a rotational body, and rotating the rotational body by their own weights.

A water tank 1 and a rotational body 20 are arranged side by side, and a plurality of buoyancy balls 10 for rotating the rotational body 20 by their own weight in a state where they are put and loaded on the upper part of the rotational body 20 are so arranged as to be circulated between the water tank 1 and the rotational body 20. Ball receiving bodies 25 on which the buoyancy balls 10 put from the upper part are to be load are continuously arranged on the periphery of a circulating member 24 to be circulated. In the water tank 1, an input return port 2 is opened and formed on the lower part, and a ball input seal mechanism 30 provided with and an input passage 31 put in connection so as to extend from the upper part of the water tank 1 to the upper part of the rotational body 20 and having the down grade, a carrying-out means 32 for carrying out the buoyancy balls 10 floating on the water surface of the water tank 1 to the input passage 31, and a sealing means 35 for allowing the buoyancy balls 10 to be rolled in the input passage 31 and shutting the inside of the water tank 1 from outside air is provided on the upper part.