PURPOSE: To improve the fitting workability of floor panels and to prevent the panels from warping.

CONSTITUTION: When a window frame 9 is fitted to a building unit 1 having a hexahedron skeleton connected to a column 3 and beams 4A, 4B, the window frame 9 is fitted by use of a horizontal framework member 10 laid between adjacent columns 3. In this way, studs interfering with the floor panels 8 are removed and the end of floor panels 8 is directly supported on the upper face of floor beam 4B by extending the end of floor panels until it reaches the upper face of floor beam 4B and hence, support tools like brackets are not required.