PURPOSE: To rotate a rotary body in order to drive a generator or the like by a method wherein lighter-than-air gas such as helium or the like is charged in a bag within a rotary vane wheel located at the lowermost position of the rotary body and discharged from the bag within the rotary vane wheel located at the uppermost position of the rotary body.

CONSTITUTION: A bag 3 is arranged within the rotary vane wheel 2 of a turbine 1. Lighter-than-air gas such as helium or the like is charged by means of fans 5 into the bag (a) of the vane wheel, which is going to reach the lowermost position of the rotary body, in order to turn the bag (a) of the vane wheel to the gas-filled bag (a') of the vane wheel. The gas-filled bag (a') of the vane wheel rotates itself due to its buoyant force until the uppermost position is reached. At this time, the lighter-than-air gas such as helium or the like is removed by means of a fan 6 from the gas-filled bag (a") of the vane wheel arrived the uppermost position, resulting in deflating the bag at (b) within the vane wheel located at the positions and also flattening the bag. The weight of the vane wheel and the air bag at the position (b) turns to be the force to lower itself.