A ship is provided with microbubble generators attached below the waterline to supply microbubbles, which decrease the ship's frictional resistance during navigation and improve fuel efficiency. The below waterline region (L.W.L.) of the ship is divided into an upper region (R1) proximate the waterline, and a lower region (R2) proximate the ship's bottom. The air supply to the microbubble generators (10) in the upper region (R1) is performed through a pipe (3), one end of which is exposed to the atmosphere. The air supply to the microbubble generators (10) in the lower region (R2) is performed through a branch pipe (6) from a pipe (5) using an assist compressor (4), which is arranged as a closed system instead of being exposed to the atmosphere.