PURPOSE: To obtain prime motive power for a machine or the like, by utilizing the buoyancy of water tanks to cause tensile forces much stronger than those of power winches for pulling the water tanks up and down, to move a balance.

CONSTITUTION: First water tanks 2 containing water are hung from both the ends of a balance 1 in such a manner that the tanks are located in first water vessels 3. Second water tanks 15, whose bottoms are connected through high pressure hoses 16 to the lower portions of the first water water vessels 3, are hung in water in the second water vessels 4. The upper and lower portions of the second water tanks 15 are linked to power winches 27 through wires 23, 24 to pull the second water tanks up and down or do the like. When the right and left second water tanks 15 are alternately moved up and down by using the winches to move the wires 23 up and down, water flows into and out of the first water vessels 3. For that reason, the first water tanks 2 containing the water and hung from the balance 1 are moved up and down by buoyancy to move the balance. Motive power is thus produced.