An arrangement for mounting in an opening of a building frame a window unit having at least one panel mounted in a rectangular framework formed by a pair of left and right longitudinal frames, and an upper transverse frame and a lower transverse frame means which are connected, respectively, to the longitudinal frames at upper and lower positions includes upper connecting metal fixtures fixedly secured to the upper part of said opening of the building frame, and lower connecting metal fixtures fixedly secured to the lower part of the opening of the building frame, the indoor side of the upper transverse frame of said window unit being connected to said upper connecting metal fixtures, and the indoor side of the lower transverse frame means of said window unit being connected to said lower connecting metal fixtures, so that a window unit can be mounted rigidly onto a building frame by a simple operation in such a manner as to prevent deformation or bending of a lower window frame and a transom towards the indoor side by the action of wind pressure etc., and that the area of the opening formed between a lower window frame of the window unit and a lower frame thereof can be increased.