To avoid contact between a ship body and water under a draft surface by providing a sheet hemming the bottom surface of the ship body and covering the whole lower surface of the ship.

A sheet formed by hemming the bottom surface of a ship is opened under water, is dropped from the front surface of the back surface of the ship, and is simultaneously pulled forward or backward by ropes 5 to cover the bottom surface of the ship. By floating members 2, 4 with the several ropes 5 attached to the sheet in the condition that the bottom surface of the ship is covered, the sheet is fixed so as to shield water. Water coming between an inner surface of the sheet and a ship body is discharged from a discharging sheet provided for the sheet when the both ends (members 2, 4) of the sheet are lifted up above the water surface. If adhesion of marine organisms onto the outer surface of the sheet is recognized, the sheet is reversed front or back. A discharging sheet 7 is also simultaneously reversed.