To improve efficiency in execution of work with welding eliminated, and to reduce cost therewith by a method wherein a reinforcing angle member is provided at a window opened to a lightweight concrete wall, and a sash is fixed to the angle member with brackets and adjustment bolts.

A bracket member 1 of a joining member 10 for installation of sash is attached between flanges 5A and 5B that are provided oppositely at an internal frame and an external frame of a sash 5 along both of the frames. The joining members 10 for installation of sash are attached to four peripheral sides of the sash 5. A fixing member 3 for each of the joining members is brought into contact with a steel angle member 7 that is fixed for reinforcement at the sash installation surface 6A of a wall 6. The fixing member 3 is turned with a spanner or the like, and positioning is made with the distance between the sash installation surface 6A and the frame of the sash adjusted to become desirable for installation of the sash 5. Then, the thrust-in part of the fixing member 3 is thrust into the angle member 7, being turned with the spanner, and an adjustment bolt 2 is fixed at this stage to the bracket with nuts 4A and 4B.