PURPOSE: To improve the external appearance of a joint part to flooring and the constructing performance by forming such protruding parts as to cover part of the flooring, at the upper part of sill side walls opposed to the end part side walls of the flooring.

CONSTITUTION: A floor panel 4 is fixed to fabric foundation 5, and flooring 2, 3 are attached and fixed into the specified positions thereon, leaving a clearance only for the arrangement of a sill 11. In this case, the clearance is formed to be slightly larger than the length between sill side walls 11b, 11c and smaller than the length between the side walls of both protruding parts 11d, 11e of the sill 11. The sill 11 is then inserted into this clearance and fixed to the floor panel 4 by machine screws or the like. The sill 11 and the flooring 2, 3 thereby present external appearance jointed neatly and continuously, and the dimensional accuracy of the sill fit-in part can be made rough so as to improve productivity.