To solve such a problem that there is only a tiny space and a complex space between a cockpit and body outer wall when a small submarine boat is manufactured so that it is difficult to arrange power generation sections, buoyancy regulating tanks, etc. of the submarine boat in these spaces compactly.
The submarine boat is made up of: the cockpit 0501 of a watertight structure; the body outer wall 0502, which surrounds the cockpit, forms a space not filled with water between the cockpit and itself, and which is designed to reduce underwater fluid resistance; gas tanks 0503 made of a flexible material arranged between the cockpit and the body outer wall; an air supplying and exhausting device 0504, which supplies and exhausts air to/from the gas tank and regulates the buoyancy that acts on the submarine boat; and the power generation sections 0505, which generate power of the submarine boat.