PURPOSE: To open or close a cover by loosening or tightening only a single nut screwed on a bolt, by a method wherein the nut is tightened from the outside of a seal plate to make the brim of an umbrella-shaped stopper expand and come in close contact with the inner surface of a drum, so that the cover is kept water-tight.

CONSTITUTION: When a cover 1 of a cylindrical vessel is closed, an umbrella- shaped stopper 5 is inserted into a cylindrical body 3, a seal plate 6 is put on a bolt 4 of the umbrella-shaped stopper 5 to mount it on the cylindrical body 3, and a nut 8 is screwed on the bolt 4 and tightened. Thereby, the brim of the umbrella-shaped stopper 5 comes in press-contact with the inner surface of the seal plate 6 and simultaneously expands outward and comes in press- contact with the inner surface of the cylindrical body 3, thus water-tightness is maintained. Since the umbrella-shaped stopper 5 is made of elastic material, a large torque is not required to tighten the nut 8 and the nut 8 is easily turned with hands without using a special tool. Therefore, the cover 1 for a battery room can be easily opened and closed, that allows a battery for a light buoy to be easily taken out front or set up in the battery room.