To effectively generate power using bouyancy motion.

Air is only initially fed from a spare pump 16 into one of lower air tanks 3 sunk in a water tank to fill the lower air tank, and therefore the lower air tank floats due to bouyancy under water. The force lifts a piston 6 of an upper air tank 4 via a toothed column 5, and air in the upper air tank is fed through a blowing pipe 9 to the lower air tank 3 in the other water tank to discharge water in it and fill air in it. The air tanks under water of the two water tanks repeat the similar motion alternately. The air pressure uses Pascal's principle, is increased as high as possible due to difference between plane areas of upper and lower air tanks, and drives a connected power generator 15 with a power transmission 12 using residual force of the bouyancy.