PURPOSE: To save the amount of using steel in beams constituting an unit box, ensure the rigidity of the beams, and also eliminate the unevenness of the floor between adjacent unit boxes.

CONSTITUTION: Between two adjacent ceiling girders 3, 3 of juxtaposed lower floor unit boxes 1A, 1A and floor girders 4, 4 of an upper floor unit box 1B placed thereon, a connecting plate 7 is interposed in the longitudinal middle part of the beams. The vertically superposed girders 3, 4 are connected to each other together with the connecting plate 7 by a bolt 10. Two reinforcing plates 9, 9 interposed in the space between the floor girders 4, 4 and the space between the ceiling girders 3, 3 are superposed and welded to the connecting plate 7. The connecting plate 7 can be fixed to the lower floor unit box 1A by a temporary fixing bolt 12 different from the connecting bolt 10 for the upper and lower girders 3, 4.