PURPOSE: To have stable input/output of objects having a high specific gravity into a storehouse by making a floating bottom plate for storage of objects in a projected form, providing divisions for accommodation of objects according to this projected form, and thereby increasing the preliminary buoyancy of the bottom plate.

CONSTITUTION: A floating bottom plate 5 for storage of objects is formed projected so as to have a greater bottom area, so that the preliminary buoyancy is increased by an amount corresponding to the wing parts. When an object is to be put in storage, the bottom plate 5 is lifted by pouring in water until the wing parts of projection is situated in the upper limit in contact with a reinforced part 11 of an apron 10. Further, the object 8 is placed upon inhibiting the buoyancy of the bottom plate by pouring water in an additional amount in consideration of the weight of object, which will hinder the bottom plate position from shifting vertically. At the time of putting out the object, the buoyancy shall be inhibited in the same manner as putting-in so that the bottom plate 5 does not rise abruptly when the object 8 moves. This construction with wing parts should facilitate control of buoyancy to accomplish stable operation.