PURPOSE: To utilize effectively water energy by providing supply and drain valves opening and closing at the upper and lower limit positions of a float on the lower portion of a cylindrical body to convert the vertical motion of the float to the rotational one and take out such motion through a one-way clutch.

CONSTITUTION: When water is supplied into a cylindrical body 1 and a float 8 is lifted, an arm 47 pivots to move upward a operating lever 43 so that a compression body 44 compresses a spring 33, while a operating lever 32 is released by a regulation releasing body 46 when an upward motion regulating body 37 is regulated to reach the upper limit position of the float 8, and moved upward so that opening and closing operation plates 6, 7 respectively lose a supply port 3 and open a drain port 5. Also, when the float 8 reaches the lower limit position, similarly a downward motion regulating body 38 is released to change over the supply port to the drain port. Thus the float 8 repeats the vertical motion which is converted to a rotational motion by a motion converting mechanism and transmitted through an one-way clutch mechanism to an output mechanism to provide power.