PROBLEM TO BE SOLVED: To eliminate a play when two seat surface portions are moved in a separating direction.;SOLUTION: On a back surface portion overlapped when an insertion flap 40 is inserted in a folding box, an insertion engaging piece 30 having a head portion 30a and a neck portion 30b is formed. The insertion flap 40 is formed with a notch 41 in which the insertion engaging piece 30 is inserted, and a pair of engaged edge portions 43 continuing to the notch 41 and extending to an end portion side of the insertion flap 40 than the notch 41. The pair of engaged edge portions 43 is formed to narrow width toward the end portion of the insertion flap 40, and the neck portion 30b is formed to increase width toward the head portion 30a. When the insertion flap 40 is extracted from the folding box, end surfaces of right and left edge portions (31) of the neck portion (30b) of the engaging piece (30) and the pair of engaged edge portions 43 are engaged with each other.;SELECTED DRAWING: Figure 5;COPYRIGHT: (C)2019,JPO&INPIT
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