A gas room is arranged in a hull to cross a longitudinal frame and to extend into a width direction. A scattering member is arranged in the gas room to cross the longitudinal frame and to extend into the width direction. Air blown into the gas room from an air supply pipe hits the scattering member to scatter into the width direction, and is blown from an air blowing hole formed in a bottom of the gas room into water. Because the gas room crosses the longitudinal frame and extends into the width direction, an air bubble flow with a wide width can be formed into the width direction by the single gas room. Therefore, the number of the gas rooms which are necessary to form the air bubble flow which covers the whole width of a ship bottom may be few, and the number of the air supply pipes which are provided every the gas room may be few.