PURPOSE: To form a rule wherein damages are concentrated on one point at a predetermined position of a fiber board by lowering a push rule under conditions that the fiber board is placed in a female die having L-shaped section.

CONSTITUTION: A part of a female die 5 corresponding to a specific push rule 2 attached to a cutting die 1 is formed into an L-shape. That is, a hard board 5 is provided only on one side of a flat plate 4 to provide a step on the plate, so that an L-shape is formed. When a fiber board 6 is placed on the die 5 and then the cutting die 1 is lowered, the board 6 is clamped between the rule 2 and the plate 4 and a card board 5, so that a rule is formed wherein damages are concentrated on one point. An edge is formed on the rule and a step is formed in a space a between the rule 2 and the board 5. When the board 6 is folded at the position of the rule, it is folded at the edge. As a result, the board 6 can be folded at a specified position.