To improve the execution for siding and facilitate the fixing of the building frame side of a roof to a frame.

A bay window has the frame 3 comprising left and right vertical frames 5, 5 fitted to the building frame of a building, mullions 6, 6, and upper frames 7, 8 on the sides and lower frames 9, 10 on the front, and a roof 4 fitted on the upper part of the frame 3 and being an almost rectangular plane. The building frame side on the side plane of the roof 4 is formed almost identical with the side plane of the vertical frame 5, and a side cap 25 for regulating the side sliding of rainwater is fitted on the building frame side plane 19 of the roof 4. On a back board 28 provided for fitting the above side upper frame 8 on the upper part of the vertical frame 5, a side back board part 29 corresponding to the side plane of the vertical frame 5 is provided, where the under part of the side cap 25 is fixed to the side back board part 29 with a fixture.