PURPOSE: To make a taking a flotage in a recovery port performable so effectively, by installing a projecting part in a lower of the recovery port of a stem part installed in a door body capable of opening or closing at will, while performing the opening or closing guidance of the on-off door body on the top of the projecting part.

CONSTITUTION: A receovery port 2 is formed in a stem part in adjoining its front, while an on-off door body, which develops and shifts outward in a beam direction of the recovery port 2, is installed therein. On the other hand, a projecting part 8 is formed in a lower part of the recovery port 2 with it expanded outward in the beam direction. Along an upper flat surface 8a of the projecting part 8, opening of closing guidance for the on-off door body 5 takes place. Thus, flotage can be led toward the recovery port 2 from the opened door body 5 in an effective manner.