PURPOSE: To reduce a dead space in the hull to the utmost as well as to make it compact in size and lighter in weight by forming such a hugging part as stickable close to the front of a diver's body, in a ballast tank being installed in the hull.

CONSTITUTION: This hull consists of reinforced plastics and so on, and a cockpit block (a) is set to the center and divided into two parts, a front block (b) and a rear block (c) as in constitution, and an air bomb 1 is installed in the front block (b) and a propulsion unit 2 in the rear block (c), respectively. High pressure air in the air bomb 1 is made feedable to each inner part of a cockpit 8 and a ballast tank 11. The propulsion unit 2 has a reversible screw 4 and a rudder 5 driven by a motor 3. In addition, each eater-filling ballast tank 11 for floating and diving is installed in front and in the rear, but this ballast tank is constituted of having such a hugging part 11' as stickable close to the front of a diver's body 7, in the intermediate part.