PURPOSE: To simplify how to mount a floor panel for a bay window and hence enhance workability.

CONSTITUTION: A prefabricated by window 1 on a module basis is laid out on the external surface of an outer wall 3 of a building 2 in such a fashion that it may project to the outdoor side. A cantilever 7 is projected from a structural material 6 of the outer wall 3 between a first floor and a second floor of the outer wall 3 of the building 2 while the cantilever 7 is used to support the bay window 1. A floor panel 32 for the bay window 1 is installed at a position higher than the cantilever 7 while the floor panel 32 is loaded on a wooden batten 47 preliminarily mounted on the inside surface of the bay window 1, thereby fixing the floor panel 32 with the wooden batten 47 by means of a fixture, such as a nail.