PURPOSE: To eliminate recessed parts even if two rules differ in thickness, with quick data entry by adding to rule data, correction data which serves to eliminate recessed parts by elongation of one rule based on the thickness of the other rule.

CONSTITUTION: If data on the position and thickness of a rule is entered from an input device 20, a rule data calculation device 22 calculates the rule data of each rule based on the position and thickness data. A rule data storage device 24 stores rule data from the rule data calculation device 22. Next, if the position of a recessed part to be corrected is indicated, a correction processing device 26 reads particular two rule data from the storage device 24. In addition, the correction processing device 26 elongates one rule based on the thickness of the other rule and adds correction data for the elimination of the recessed part to rule data. Rule data which has undergone correction processing is again stored in the rule data storage device 24.