To provide a structure of drainage from the upper surface of the lower frame of a flat bottom sash, which smoothly drains water leaking out to the indoor side on the upper surface of lower frame from the lower sash meeting part of horizontally sliding windows, where a hang-down piece is hung from the bottom sash of double sliding screens and fitted into a slide groove formed at the upper surface of each bottom window frame.

The bottom window frame for an external opening of a building has a structure where hand-down pieces are hung from outside the bottom frames of outer and inner horizontally double sliding windows provided at this external opening, and fitted into outer and inner window slide grooves formed at the nearly flat upper surface of the bottom window frame. The upper surface of the bottom frame located between the slide grooves of outer and inner windows is formed using an upper surface member, and drain ditches inclining from the upper surface of bottom frame toward the inner window slide groove is formed at the indoor side of this upper surface member at a position near the sash meeting part. It is better to form a plurality of these drain ditches nearly in parallel.