PURPOSE: To raise the watertight performance by a method wherein a movable frame elevating in parallel with a lifter and engaged with the packing is provided in the peripheral gap between the opening of an upper deck and the lifter moving up and down between the upper and lower decks and giving the watertightness to the opening of the upper deck when it is housed.

CONSTITUTION: A lifter 10 moves vertically and is placed in a housed condition in the position where it closes the opening of an upper deck 11. When it is housed, the lifter is fixed by a lifter support 29 and placed in a watertight condition through the fitting of tightening bars 17a and 17b to the packings 18a and 18b under the pressure due to the lowering of a movable frame 16 through the intermediary of a link mechanism 21. When the lifter 10 is employed, the movable frame 16 moves upward by the reverse operation of the link mechanism 21, thereby the above fitting under pressure being released and the lifter 10 being place in the condition that it can be moved up and down freely.