Abstract: In a state where definition of machining has been already finished for the first to N-th processes, such data as a machining kind, tool data including a tool code, and cutting conditions related to the j-th process (1.ltoreq.j.ltoreq.N+1) are inputted as new data or modified data in an interactive mode on a screen. Next, a shape of the workpiece at a point of time when the (j-1)th machining process has been finished and the final shape of the workpiece to be cut are displayed on a screen, and a cutting area for the j-th machining process is set or modified by means of the screen. Then, a shape of the workpiece at a point of time when the j-th process is finished is found on the basis of the inputted data, and a cutting area to be newly cut or to be modified in the j-th machining process is identified and displayed by displaying one upon the other on the screen the found shape of the workpiece and a shape of the workpiece at a point of time when the (j-1)th process has been finished.