A fence 30A is installed on an inner surface 21a of a door 20A on the side which faces a landing gear bay 15, the fence 30A rising inwardly of the airframe from the inner surface 21a. When the door 20A opens outwardly of an airframe due to a pressure difference between the side of an outer surface 21c of the door 20A and the side of an inner surface 21a facing inner part of the landing gear bay 15 during a flight of an aircraft 10, the fence 30A closes a gap between the door 20A and an opening 15a of the landing gear bay 15, and thereby prevents air from flowing into the landing gear bay 15 through the gap.