PURPOSE: To shorten term of works while reducing costs including labor cost in construction of a unitized building, and to prevent obstructions from occurring in stabilization of quality.

CONSTITUTION: A fiber-reinforced plastic waterproof pan 21, equipped with an upright peripheral wall part 22, is installed on beams 4 and 5 (joists for an entry unit) positioned to the interior side of an opening for an entry that is formed to a structure body 1 (a second story entry unit). Wooden rails 31 are provided to the outer periphery of the upright peripheral wall part 22 of the waterproof pan 21. Waterproof members 37 are provided between the peripheral parts 33 of the rails and the surrounding erected wall part 22 of the waterproof pan 21 while the waterproof members 37 are provided between a door sill 7 to be provided to the opening for the entry and the upright peripheral wall part 22 of the waterproof pan 21.