PURPOSE: To provide an energy generating apparatus, an alternative to internal combustion engines, to obtain ecological energy which has been urgent today.

CONSTITUTION: Buoyancy is used to rotate a rotary disk 1 to obtain energy. To obtain buoyancy half part of the rotary disk 1 is placed in liquid in a liquid chamber 2 with the rest of the disk 1 in gas in a gas chamber 3. When buoyancy is exerted on the part of the rotary disk 1 in the liquid to lighten this disk, it loses the balance between the disk in the liquid and that in the gas, rotating the disk 1. The rotation continues until both of the liquid in the liquid chamber 2 and the gas in the gas chamber 3 are lost.