PURPOSE: To execute the replacement of a sash simply and rapidly by removing an existing frame from a mounting wall body, connecting a fixture exposed to an existing-frame mark and a fixture set on the rear side of a new frame and fixing a new sash to the mounting wall body.

CONSTITUTION: When an existing frame 1 is removed, the base section of a fixture 7 is left on a sash-mounting wall body 6, and the front end section of the base section is exposed to the existing-frame mark of the wall body 6. A reinforcement 8 is welded to the exposed fixture 7, and the reinforcement 8 and a fixture 10 set on the rear side of a new frame 9 are connected by the welding, etc., of a connecting reinforcement 11, and the new frame 9 is fixed temporarily onto the wall body 6. A clearance between the new frame 9 and the wall body 6 is filled with mortars 13, and the covering material of a casing 14, a throating 15 or the like is installed along the new frame 9 while a necessary section is caulked and finished.