To simplify execution and improve watertightness by fitting a veranda erected with fence walls on the edges of a boat shape (tray shape) provided with rise walls on a bottom board to the floor joist frame of a skeleton, and fitting back boards on the inner side of the fence walls.

Fence wall units 2A, 2B are watertightly fitted to the upper edges of a boat-shaped bottom frame 1 integrally formed with rise walls 1b-1e on the periphery of a rectangular bottom board 1a, and a drain section is provided on the bottom board 1a to form a veranda. This veranda is fixed on a skeleton side floor joist frame via floor joist bars 10a and a floor joist board 10b formed at a downward grade to the outdoor side. A waterproof material such as asphalt roofing or polyethylene fill is installed between the floor joist board 10b and the boat-shaped bottom board 1a, and a sealant is filled at connection portions such as bolt holes to watertightly fix them. Back boards 6 are fitted on the inside of fence walls as a blindfold. The veranda is efficiently fitted, and the watertightness can be improved.