To fold all the side walls to compactly collect them into the size of a bottom wall by folding a pair of side walls with hinges by bringing them into contact with the top face of the bottom wall and folding another pair of side walls by bringing them into contact with the top faces of the folded side walls.

A pair of side walls 2a and 2b are connected with a bottom wall 1 integrated with auxiliary bottom wall members 5 with hinges 4 existing along the top face end edge of the bottom wall 1. It is possible thereby to fold the side walls 2a and 2b by bringing them into contact with the top face of the bottom wall 1 with hinges 4. Auxiliary bottom wall members 10 are connected with the inner face side end edges of another pair of side walls 2c and 2d and the outer face side end edge of the bottom wall 1 with hinges 8 and 9 to form rising parts. The height of this rising part is set in such a way that the side walls 2c and 2d are brought into contact with and foldable to the top faces of the side walls 2a and 2b being brought into contact with the top face of the bottom wall and folded. It is possible thereby to collect all the side walls 2a-2d to the size of the bottom wall 1.