In a ship-body frictional resistance reducing device, an ejection chamber having an air ejection hole is fixed to a bottom of a ship body, and an air supply path for supplying air to the ejection chamber and a marine-biofouling preventing-fluid supply path for supplying a marine-biofouling preventing fluid to the ejection chamber are connected to the ejection chamber. An on-off valve 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.