PURPOSE: To improve weakness occurring to periphery of a sash when fitting the sash to the exterior side on the periphery of an opening by making it flush with the finished surface of an external wall by a method wherein watertight structure is provided between the sash and a building frame.

CONSTITUTION: A waterstop 50, made up in a L-shaped section, is fixed along the periphery of an opening 30 provided to concrete 23 on building frame in the form of external wall 2 or the like and along its interior-side periphery, covering three sides of the opening except its bottom side. A fixing member 54 in square-shaped section is fixed in projection to a piece in the direction of depth 52 of the waterstop for fixing an anchor 46 of a sash 4.