A frictional resistance reducing device of a ship, comprising: a gas room provided in a hull to cross a longitudinal frame and to extend into a width direction of the hull; a scattering member arranged in said gas room to cross said longitudinal frame and to extend into the width direction; an air supply pipe connected with said gas room, wherein the arrangement is such that air blown into said gas room from said air supply pipe hits said scattering member and is scattered into the width direction and then blown from an air blowing hole provided in a bottom of said gas room into water, wherein a first opening (31a;33a) is provided in a portion of said gas room on a port side from said longitudinal frame to connect an internal space of the hull and an internal space of said gas room, wherein a second opening (31b;33b) is provided in another portion of said gas room on a starboard side from said longitudinal frame to connect the internal space of the hull and the internal space of said gas room, wherein a first lid (35;37) is provided for said first opening (31a;33a), and a second lid (36;38) is provided for said second opening (31b;33b), wherein a first part of said scattering member which is a part of said scattering member on the port side from said longitudinal frame, and a second part of said scattering member which is a part of said scattering member on the starboard side from said longitudinal frame can be separated from the scattering member, and wherein said scattering member first part has such a size that said scattering member first part can pass through said first opening (31a;33a), and said scattering member second part has such a size that said scattering member second part can pass through said second opening (31b;33b). a gas room provided in a hull to cross a longitudinal frame and to extend into a width direction of the hull; a scattering member arranged in said gas room to cross said longitudinal frame and to extend into the width direction; an air supply pipe connected with said gas room, wherein the arrangement is such that air blown into said gas room from said air supply pipe hits said scattering member and is scattered into the width direction and then blown from an air blowing hole provided in a bottom of said gas room into water, wherein a first opening (31a;33a) is provided in a portion of said gas room on a port side from said longitudinal frame to connect an internal space of the hull and an internal space of said gas room, wherein a second opening (31b;33b) is provided in another portion of said gas room on a starboard side from said longitudinal frame to connect the internal space of the hull and the internal space of said gas room, wherein a first lid (35;37) is provided for said first opening (31a;33a), and a second lid (36;38) is provided for said second opening (31b;33b), wherein a first part of said scattering member which is a part of said scattering member on the port side from said longitudinal frame, and a second part of said scattering member which is a part of said scattering member on the starboard side from said longitudinal frame can be separated from the scattering member, and wherein said scattering member first part has such a size that said scattering member first part can pass through said first opening (31a;33a), and said scattering member second part has such a size that said scattering member second part can pass through said second opening (31b;33b).