In the present invention, there is a first inclined line (18) which extends over a first wall (12) toward a third wall (44) side from a branching point (16) on a folding line (15) between the first wall (12) and a second wall (13), a second inclined line (19) which extends over the second wall (13) toward the third wall (44) side from the branching point (16), and a cutout part (24) which extends from the tip of the first inclined line (18) to the tip of the second inclined line (19), and between the abovementioned elements is a locking-receiving part (23) which will extend at a slope over the first wall (12) and second wall (13) at assembly. A locking part (48) for engaging with the cutout part (24) side of the locking-receiving part (23) is provided on the third wall (44), which will be disposed along the inner surface of the second wall (13).