PURPOSE: To produce power by providing a liquid circulation path conducting with a cylinder in a container to conduct between a cylinder chamber and the container through a valve while inserting a piston associated with a spring.

CONSTITUTION: A liquid circulation path 14 conducting with a cyliner 15 is provided in a container 11 to conduct between the cylinder 15 and the container 11 through a valve 12, while a piston associated with a spring 17 is inserted into the cylinder 15. Then water is filled in the container 11 to the position 21. The gravity applied on the piston 16 when the valve 12 is opened is higher than when the valve 12 is closed. Consequently when the valve 12 is opened, the piston 16 will move by the gravity to the direction of arrow 19 while when closing the valve 12 it will move by the force of the spring 17 to the direction of arrow 20 thus to produce the power.