PURPOSE: To allow a casing to be fitted to a window hole rapidly with a high accuracy by connecting the casing, framed in rectangular shape, integrally to a window frame, and fixing the casing to the strength member of the window hole by a fitting flange and the bracket of the casing.

CONSTITUTION: The angle piece of a window frame 16 is fitted to the outer end part inner surface of a casing 17 and fastened to the outer end part by a set screw 20 for integral connection to form a window frame unit 15. The window frame unit 15 is fitted to the window hole 2 of a building and fixed to a strength member 3 by driving a fastener 21b from the inside. Exterior wall material 14 is fitted to the outer face of outer furring strips 8, and sealant 14a made of modified silicon is filled in a gap part between the window frame 16 and the window hole 2 to prevent the infiltration of rainwater. Inner furring strips 12 are fitted to the inside of a column 3. Interior wall material 13 is fitted to the inside of the inner furring strips 12, and the end part of the interior wall material 13 is fittingly locked to the locking groove 17a of the casing 17. The window frame 16, the casing 17 and a bracket 18 are thereby assembled easily in a factory, and the assembled window frame unit 15 is positioned in the window hole 2 so as to make fitting work easy and rapid.