PURPOSE: To install a molding extending from the extrusion-molded part to the upper and lower molded parts and to simplify the manufacture and after- treatment therefor.

CONSTITUTION: A molding installation groove 12 extending from an extrusion- molded part 7 to the upper and lower molded parts 8 is formed in nearly the central area of a seal surface 3 of a vertical side 2. A masking grip 13 which can be opened and closed and cut off is integrally formed on the groove edge of the molding install groove 12. In the after-treatment after manufacturing, the surface treatment is performed on the seal surface 3, with the molding install groove 12 closed by the masking grip 13. After that, the masking grip 13 is cut off and a molding is installed in the molding install groove 12.