PURPOSE: To prevent the incapability of vertical movement of a frame part due to flexion of oil-preventive films by a method in which a frame part is attached in a vertically movable manner along the guide of a restraining part, and the end of a soft oil-proof film is attached in a relatively rocking manner to the opening curved face of the frame part.

CONSTITUTION: A guider 13 is fixed to a fixed column 11 erected from the seabed, and a frame part 18 is provided in a vertically movable manner through rollers 14 and 15 to the inside of the angular groove of the guider 13. A lip seal 11 is attached to the fixed column side of the frame part 18, and one end of a soft oil-proof film 12 is inserted into and supported on the curved face 18a of the opening of the frame part 18. Even when the film 12 is flexed by pressures of water, the flexion is absorbed by the curved face 18a without acting on the frame part 18. The frame part 18 can thus be vertically moved along the guider 13 smoothly all the time in comply with variations of tidal levels.