PURPOSE: To obtain the specified thickness of a score line and a stable opening property by providing a lower die with a 2nd forming part having the front end width wider than the width obtd. by adding a deviation size to the bottom part width of a 1st forming part.

CONSTITUTION: The 2nd forming part 11 has the front end 13 wider than the width obtd. by adding the deviation size to the bottom part width w2 of the 1st forming part 6 and, therefore, the bottom part 9 of the 1st forming part 6 is prevented from deviating from the range at the front end 13 of the 2nd forming part 11 facing the above-mentioned part so that the corresponding states thereof are maintained each other. The thickness of the score line 5 is constant over the entire periphery thereof and the stable opening property is obtd. at the time of opening along the score line 5.