To provide an assembled structure of box capable of easily locking its bottom plate configuration piece through one step without applying any glue to a flap-like bottom plate configuration piece forming a bottom wall.

A blank is comprised of four side plates continuous in right and left longitudinal directions; four bottom plates continuous to each of the side plates at any one of longitudinal and lateral directions; and a lid plate continuous to either all the side plates or a part of each of side plates. The four bottom plates have a trapezoidal part with its lateral width being narrowed from each of the side plates toward the extremity end and a lock piece protruded at the extremity end. The trapezoidal part is inclined at one side of the right and left sides from one end of the bottom side becoming a cooperative side with the side plates onto an extension line at a substantial central position of the bottom side and at the same time it becomes an inclined side in which the longitudinal width acting as a height is set to a substantial half length of the bottom side of the side plate adjacent to the side plate having the trapezoidal part cooperatively connected.