An energy conversion device according to the present invention comprises: a
liquid tank; a gas receiving part which can rotate or move in a vertical direction, a plurality of which are provided along the lengthwise direction inside the
liquid tank; an input part which receives input of a substance serving as a
primary energy source in the lower portion inside the
liquid tank; an output means which outputs, to the exterior of the liquid tank as
secondary energy,
kinetic energy for rotation or upward movement that occurs in the gas receiving parts as a result of
buoyant force occurring in the gas receiving parts upon receiving a gas corresponding to the substance; and a
recovery device that recovers the substance from the liquid tank and returns the substance to the input part. The liquid tank includes a first liquid tank and a second liquid tank. The energy conversion device comprises: a
pipe which guides gas that accumulates in the upper portion of the first liquid tank to the second liquid tank; a
buffer tank which is connected to the
pipe; an on-off valve which is disposed at the inlet and outlet of the
buffer tank; and a pressure adjusting mechanism which detects the
internal pressure of the upper portion of the first liquid tank and the
internal pressure of the
buffer tank, and adjusts the pressure of the gas input to the lower portion of the second liquid tank by opening and closing the on-off valve in accordance with the detected internal pressures.