To connect stiles and rails which are formed into a rectangular pipe shape to form into a frame, with high working efficiency.

The rails 9 are formed to have a plate thickness, with which the stiles can be inserted into the cylinders of the stiles 6, 7; meanwhile Slits 6g, 7g, 7h, 8f are formed in the side pieces 6c, 7b, 7c, 8b of the stiles 6, 7, 8 excluding the outer pieces 6b 8c thereof on the door front side and the door rear side; and the cylindrical ends of the rails 9 are inserted into the cylinders of the first, second, and third stiles 6, 7, 8 via the slits 6g, 7g, 7h, 8f and are fixed to the outer pieces 6b 8c of the first and third stiles 6, 8 and the cylindrical ends of the rails 9.