A winding roller for winding and unwinding a corner projected canvas and a rectangular canvas in such a state that the canvases overlap is supported via a bearing and a slider for attaching a projected corner canvas to the winding roller and making the canvas freely slidable is incorporated. The front bar for the rectangular canvas is supported by a foldable arm in such a manner so as to be freely translatable and the front bar for the projected corner canvas is formed in the front bar for the rectangular canvas in such a manner so as to be freely slidable. Thus, the projected corner canvas and the rectangular canvas, which are wound around the winding roller in such a state that the canvases overlap are unwound so as to spread, and the spread projected corner canvas slides transversely along the winding roller and the front bar of the rectangular canvas, and thus, extends to the outside of a building which includes a corner space portion of a projected corner portion. The projected corner canvas which extends into the corner space portion transversely slides to the rear along the winding roller and the front bar of the rectangular canvas while remaining in a spread state, and after that, the projected corner canvas and the rectangular canvas are wound around the winding roller in such a state that the canvases overlap.