To provide a relatively tough and durable box being assembled by bending one cardboard cut into a predetermined shape and friendly to environment.

Outer wall parts 7, 11, upper end surfaces 4, 13, inner wall parts 8, 12 and adhesive parts 10, 15 are provided in the respective sides 17, 18... of a rectangular bottom surface part 1 through fold lines and reinforcing parts 16, 16 bent into a horizontal U-shape are formed in both sides of the outer wall parts 1, 11 constituting both opposed side walls 3, 3 and the paper pieces formed in the respective sides of the bottom surface part 1 to constitute side walls 2, 3... and the reinforcing parts 16, 16 are housed in the internal spaces 9, 9 of the adjacent side walls 2, 2 and holes 19, 19... are allowed to pierce along the sides 17, 17 of the bottom surface part 1 and fitting parts 5, 5... are protruded from the upper end surfaces 4, 4 of the side walls.