PURPOSE: To make sure waterproofing.

CONSTITUTION: A penetration hole 24 is installed to the top of a handrail, which penetrates a floor panel 22 to which vinyl chloride-clad steel plate 22c is installed, which is a water proofing layer. A mounting member 50 is fixed with the penetration hole 24 while the mounting member 50 is projected upward from the penetration hole 24 as well. A handrail post 41 is mounted to the mounting member 50. A flange 53, whose size is larger than that of the penetration hole 24, is installed to the mounting member 50 where the flange 53 and the vinyl chloride-clad steel plate 22c is joined by heating with a deposit metal S, thereby sealing the area between the flange 53 and the vinyl chloride-clad steel plate 22c.