To obtain CL(cutter location) data and NC data immediately and to improve operation efficiency when a mathematical expression representing a work surface is changed by inputting the mathematical expression representing the work surface and its partial differential expression and calculating CL data indicating a tool position.

The X and Y partial differential mathematical expressions of the mathematical expression representing the work surface are found first and inputted from a keyboard 13 together with the mathematical expression representing the work surface. The inputted mathematical expression data are stored on a hard disk 12d. Then a program stored in a ROM 12b automatically corrects an CL data arithmetic program according to the mathematical expression data. The corrected program is executed to calculate CL data, and NC data is calculated from the CL data. The found NC data is inputted to a numerical controller 10 from an interface 12g to machine a metal mold 26. Then machining can be started by an NC working machine immediately after the mathematical expression of the work surface is given.