PURPOSE: To prevent a long frame member, an upper side wall, a lower side wall and a long bottom wall from sliding aside by a method wherein protrusions are fitted in recessed parts when the upper side wall, the lower side wall, etc., are jointed together.

CONSTITUTION: When a folding box is assembled, a protruding stud 5c provided on the upper part of the outer face of an upper side wall 5 is fitted in a recessed part 1e provided almost in the lower middle of the inner-face of a long frame member 1. A protruding stud 5d provided almost in the lower middle of the outer face of the upper side wall 5 is fitted in a recessed part 6c provided on the upper part of the inner face of a lower side wall 6. Furthermore, a protruding stud 6d provided almost in the lower middle of the outer face of the lower side wall 6 is fitted in a recessed part on a long bottom wall. Therefore, even when a side impact force is imposed on an upper frame, for example, the long frame member 1, the upper side wall 5, the lower side wall 6 and the long bottom wall are prevented from sliding aside, so that the joints between the long frame member 1, the upper side wall 5, the lower side wall 6 and the long bottom wall which compose the folding box can be certainly prevented from breaking.