To provide a multi-panel window sash that can surely receive dew condensate of a mullion to drain it to the outdoor side, in a sash for a building in which sashes laterally adjacent at an angle to each other are connected through the mullion.

A vertical frame 11 includes an indoor projecting piece 11a and an angle piece 11b. An indoor part 24 of the mullion 3 has side parts 25 parallel with respective facing surface parts on both sides, and a center face part 27 provided between the side parts at a specified angle to each side part. The indoor side of the mullion 3 is provided with a drain guide component 30 at the lower end. The drain guide component 30 has a center receiving part 31 receiving dew condensate on the upper face along the center face part 27, and approximately L-shaped guide parts 34 extended along an angle piece 11b and the indoor projecting piece 11a of the vertical frame 11 from both sides of the center receiving part 31. The guide part 34 has a drain passage 35 of inclined face shape at the upper face to guide the dew condensate received by the center receiving part 31 to the outdoor side of the indoor projecting piece 11b, and the tip part of the guide part 34 faces the outdoor side.