PURPOSE: To enable sewing by a sewing machine and to enable folding so as to facilitate conveyance by connecting the connecting portions of the respective plates to each other by two pieces of connecting cloth or a fastener to be sewed on plates of connecting cloth, sewing a fastener on the plate, and then sewing two separated pieces of cloth to each other.

CONSTITUTION: One side of each side plate 12 is sewed to each of four sides of a bottom plate 10 through a connecting cloth 14 comprising two pieces of cloth, and a fastener is sewed extending over the side plates 12. That is, first piece of cloth 14a is sewed to each side of the bottom plate 10, and second piece of cloth 14b is sewed to one side of each side plate 12. Secondly, pieces 16a, 16b of fastener cloth are sewed to one pair of adjacent side plates 12. Subsequently, the free edge of the first cloth 14a and the free edge of the second cloth 14b are sewed to each other.