PURPOSE: To prevent the width of a stair well for a stair from becoming narrow, by locating the stair between body floor panels attached to an upper floor, and by supporting a stair top floor panel located above the stair with joint beams laid below the body floor panel.

CONSTITUTION: A stair top panel 2 is laid between body floor panels 55, 55 above a stair, channel shape joint beams 1 having opposite ends which are fixed to end plates, are laid below the body floor panels 55 between a barrel difference 53 and a floor beam. Further, the upper floor panels 2 are supported to the joint beams 1 through the intermediary of a Z-like fixture 6. With this arrangement, the width of a stair well for the stair 50 can be widened by a width corresponding to the flange parts of the joint beams 1, 1.