PURPOSE: To reduce the frictional resistance of a ship by jetting air from its discharge port at the ship's bottom and forming an air layer along the outside of the ship's bottom and the outside of the side board under the waterline.

CONSTITUTION: Protruded stripes 5 and 6 are provided at a ship's bottom in the longitudinal direction and air is jetted from its discharge port 4 toward the section made by these protruded stripes 5 and 6. An air layer is formed in the section made by the ship's bottom and the protruded stripes 5 and 6 and the frictional resistance of the ship can be reduced.