1. A double-floor member comprising:at least one floor panel comprising an upper surface configured to be substantially flat and square shaped forming a horizontal floor face held horizontally at a predetermined height from a foundation floor face, said at least one floor panel being arranged side by side on which at least one case is adapted to be placed;
at least one first guide rail that is directly in contact with and fixed to the upper surface of said at least one floor panel, said first guide rail being extended in a horizontal direction along the upper surface of the floor panel ; and
at least one first connecting member that is movable in a longitudinal direction of said first guide rail and is engaged with said first guide rail so as not to be separated upward from said first guide rail;
wherein said at least one first connecting member is configured to be connected to said at least one case by at least one fixing bolt so that the first guide rail and the first connecting member are not directly in contact with the case.