In a ship-body frictional resistance reducing device, an ejection chamber (13a, 13b, 13c, 13d, and 13e) having an air ejection hole (14a, 14b, 14c, 14d, and 14e) is fixed to a bottom (12) of a ship body (11), an air supply path for supplying air to the ejection chamber (13a, 13b, 13c, 13d, and 13e) and a marine-biofouling preventing-fluid supply path for supplying a marine-biofouling preventing fluid to the ejection chamber (13a, 13b, 13c, 13d, and 13e) are connected to the ejection chamber (13a, 13b, 13c, 13d, and 13e), and an on-off valve (23, 34) is provided on each of the paths, by which either one of the paths can be connected in a selective manner, making it possible to prevent adhesion of marine species to the air ejection hole of the ship body while suppressing power consumption therefor.
