To prevent a backboard from being curved without using thick cardboard or without extending a folded back piece by bending both sides of the backboard backward, and fixing the bent state with shelf boxes constituted of floor boards and frame boards.

For the structure of a shelf box 7, a floor board 2 is located in the perpendicular direction to bend lines 4, and the back edge of the floor board 2 is arranged along the front face of a folded backboard 1. The lower section of a frame board 3 is arranged along the front and the edges on both sides of the floor board 2 and is stuck to the lower face of the floor board 2 by adhesive pieces provided on the lower side of the frame board 3. Adhesive areas 5 extended on the back edge of the frame board 3 via fold lines are adhered to segment faces on both sides of the backboard 1, and the tip edges of partition boards 6 extended at the tip edges of the adhesive areas 5 via fold lines are adhered to the insides of the front area of the frame board 3. Both sides of the backboard 1 are folded backward at the bend lines 4, the backboard 1 is reinforced, the volume feeling and PR degree are increased, and the folded state of the backboard 1 is fixed by the shelf boxes 7.