To keep a right-angled state by folding a corrugated fiberboard sheet at rules orthogonal to corrugations.

The rule structure has an approximate U-shaped or H-shaped protruding plane 3 comprising two cuts (a) crossing a crease 2 and a cut (b) connecting between ends of the cuts at one of sides as a fold 5 for folding a corrugated fiberboard sheet 1 at the crease 2 orthogonal to corrugations. A plurality of the protruding planes 3 are continually provided along the crease, and the fold of the corrugated fiberboard sheet with a length from the crease 2 of the cut (a) to the cut (b) which is less than a thickness of the corrugated fiberboard sheet is formed.