To provide a transom material shielding a heat between indoor and outdoor rails, preventing the generation of a thermal warpage and making extrusible and having a little depth while keeping a balance in the strength of internal and external materials.

The indoor side and the outdoor side are composed of aluminum profiles 10 and 20 respectively, and a transom 1 consisting of a heat insulating sash heat-insulating a section between the aluminum profiles by a resin material 30. A reinforcing material 40 remarkably increasing the strength of the outdoor- side aluminum profiles 20 is mounted on the outdoor-side aluminum profiles so as not to be brought into contact with the resin material 30 and the aluminum profiles 10.