To discharge the drainage from vertical joints separately from horizontal joints by arranging drain pieces at intersections between vertical and horizontal joints, and suspending the drain pieces on the right and left horizontal joints crossing the vertical joints.

The recessed stripes 13 of the upper frame 6 and the protruded stripes 14 of the lower frame 7 of upper and lower adjacent sashes are coupled to provide horizontal joints between the recessed stripes 13 and the protruded stripes 14, and the vertical frames 9 of the adjacent right and left sashes are fixed to a substrate steel structure. Drain pieces 40 are arranged at intersections between horizontal and vertical joints, the continuation of the horizontal joints is cut off for each horizontal line of the sash, and the rainwater of the vertical joints is prevented from flowing into the horizontal joints by cutoff walls 44a provided on the drain pieces 40. Water run sections at the coupling sections in the vertical direction of the sash are separated from the vertical joints by water run preventing walls 42a to prevent rainwater from flowing into the vertical joints from the water run sections, and the condensed dew in the sash dropping from the lower ends of the vertical frames 9 is collected in condensed dew receivers 49 and discharged to the outside from discharge ports 45a, 45c. The rainwater flowing in the vertical joints can be prevented from flowing into the horizontal joints and entering indoors.