PURPOSE: To reduce a manday at a site and substantially improve working efficiency by prefabricating mounting base frames in units for a bay window separately from a building body and fitting the base frame units to the building body.

CONSTITUTION: A bay window mounting base frame 1 has an opening frame part 18 between right and left formwork members 2 and 3, and horizontal frame members 5 and 6, and a bay window base frame part 8 is provided at the lower side of the opening frame part 18. The aforesaid mounting base frames 1 are prefabricated in units, lifted with a crane and the like and brought from the outside to a building body. Then, the frames 1 as they are fastened to the building body with a bolt and the like. For the mounting base frame 1, a bay window body 26 comprising a roof part 24 is fitted to a window part 25. According to the aforesaid construction, it is possible to remarkably reduce a manday at a construction site and simplify a fitting work.