PURPOSE: To prevent the resistance of ship from being increased and an air ejecting port from being clogged due to adhesion of marine creatures by a method wherein the air ejecting port is provided at an inside part of a plug which can be protruded outboard, and when the plug is retracted inboard, the ejecting port is shut up by a sealing part and the outside surface of the plug is made to be flush with the outside surface of the hull.

CONSTITUTION: A cylinder 3 is fitted to an outside plating S through a boss 2. A piston 4 having at its tip a plug part 4a which can be protruded outboard is placed in the cylinder 3. When the plug part 4a is protruded outboard, air for forming a masking bubble layer is ejected through an air passage 5 formed at a central part of the piston and the air ejecting port 5a provided at an inside part of the plug part 4a. When the masking bubble layer is not required, the plug part 4a is contained in the hull so that the resistance of the ship is not increased and the adhesion of marine creatures to the ejecting port 5a can be prevented.