The present invention has been relatively weight is provided to transport by housing the object of heavy fruit or the like, in particular, yet give strong support means that can be sustained on its own for the pressing force to be concentrated in the vertical downward to the sense of security protection of the receiving object to deploy free and easy than in a closed box assembly is possibly because this is to relieve the pain of associated workers, of course, folded to be able to achieve the maximization of productivity through the improvement of working methods and easy handling paper box structure, and it the relates to a method for assembling utilized, the front plate and the side plate are respectively the upper plate 21 and lower plate 22, an upper side plate 31 and the lower side plate (32) separated include knitted and the lower plate (22) employing a support reinforcement longitudinally opposite ends 23, and employs a mounting fly 24 that is a length of extension than whether reinforced with both ends of the upper plate (21), each mounting fly has the mounting of the upper plate fly adjacent but given a fixed gap (25) to be mutually bound, by placing the mounting fly so that the direction opposed to each other in the upper plate ends fly mounting of the upper plate which is the next to one another as given differently the incision direction of the fixed gap It characterized in that it is presented together mutually the same shape.