PURPOSE: To produce many separate detained gases in water at a time by regularly arranging many polygonal hollow cylinders in the longitudinal direction to form a flat plate, adhering the bag membrane to the upper surface of the flat plate, horizontally setting the assembly in water, and sending gas into the bag and cylinder.

CONSTITUTION: When the gas-impermeable membrane is adhered to the circumferential edge of the upper plate of the perforated flat plate having many polygonal hollow cylinders, the bottoms of the cylinders are opened at the lower plate of the flat plate. The gas-impermeable membrane is used as a part of the bag, a breather pipe A piercing the membrane and capable of sending gas into the bag is set, and further a breather pipe B piercing the membrane covering the flat plate and capable of sending gas into the cylinder is set. The device is horizontally held in water, gas is firstly sent through the pipe B to fill the cylinder with gas at a stretch. Gas is then sent through the pipe A to expand the bag until the openings of the cylinders on the upper plate are pressed by the bag. By this method, many separate detained gases can be produced in water at a time. The device can be utilized in the transportation of live fish by a vehicle.