PURPOSE: To promote working efficiency by forming an opening in a wall material, at the same time, providing reinforcing frames in the rear of the wall material so that they are located on both sides of the opening in a state to expose a part of them from the opening, and fixing a window frame to an exposing section from the front side.

CONSTITUTION: An opening 7 is formed in a wall panel 1 of a wall material 2 such as a hard wooden piece cement board, etc., to which reinforcing frames 3, 4 and 5 of aluminum push formed sections, etc., are mounted. After that, the frames 3 and 4 are arranged to both sides of the opening 7 in a state to expose a part of them from the front side through the opening 7 to enable sighting. Then, a window frame 6 is fitted in the opening 7 from the front side, and the window frame 6 and an exposing section of the frames 3 and 4 are fixed with rivets, etc. After a gasket 8 is inserted in joints S1 and S2, the panel 1 is mounted on a stud 11 as a column material through the frames 3 and 4, and the same substance as the gasket 8 is inserted in a joint S3 between the panel 1 and an adjoining panel 1X. According to the constitution, the window frame can be easily mounted, and complete waterproofing between the wall material and window frame can be executed.