Abstract: A water driven power system has a cylindrical body with valved water supply and drain ports in the bottom thereof, the valves including valve plates rotatable between valve open and valve closed positions. A float is freely movable up and down between an upper limit position and a lower limit position in the cylindrical body due to buoyancy during supply of water through the supply valve and due to gravity during drainage through the drain valve. The float movement during the time of water supply is converted to rotation and a one-way clutch mechanism supplies the motion to an output mechanism. A valve opening and closing mechanism closes the water supply valve and opens the drain valve at the upper limit position of the float and opens the water supply valve and closes the drain valve at the lower limit position of the float.