PURPOSE: To eliminate any necessity for water-tightness and air-tightness further enable sinking and floating operation to be perfectly controlled in a short time, by liftably arranging only a ship body supporting bed for a buoyant unit.

CONSTITUTION: A ship body 2 is introduced to a prescribed mark position of the center of gravity by using a fixing lever additionally used as a guide lever 7 and fixed by causing a suitable cable or the like to prevent the ship body from its longitudinal movement, on a ship body supporting bed sunk to a predetermined depth. Next, a winch 23 is connected to a power supply on the ship body 2 by a lead wire 24, and if the winch 23 is started to wind the cable, the cables 16, 17 are pulled lifting the ship body 2 to the position of a perfect upper mount integrally with the ship supporting bed. While the cable having fixed the ship body 2 to the ship supporting bed is unwound, and a stopper is released, then if the cables 16, 17 are loosened by the winch 23, launching is completed by lowering the ship body 2 and the ship body supporting bed altogether in a fixed distance and detaching the ship body 2 from a ship body supporting receiver 6 on the ship body supporting bed.