Micro-bubble generators attached to a ship hull below the waterline decrease frictional resistance during navigation and improve fuel efficiency. Each micro-bubble generator is supplied with air and has a wing positioned outside the hull for generating negative pressure. The negative pressure created by the wing increases with an increase in ship's speed, which pushes the air-liquid interface outwardly toward the wing. As the air and the water move at different speeds due to difference in their densities, micro-bubbles are generated in the micro-bubble generator according to the Kelvin-Helmholtz Instability mechanism and discharged downstream to move closely along the outside surface of the hull.