PURPOSE: To eliminate an uncut part by correcting the coordinates of a closed loop end point in closed loop data constituting the outer periphery of a product in a desired shape on a plate to be machined by a data processing part.

CONSTITUTION: Outward appearance machining data consisting of machining figure data and machining tool data after being inputted to a data input part 1 and temporarily stored in a data storage part 2 are passed to the data processing part 3 in sequence. This processing part 3 judges whether or not the passed outward shape machining data shows the closed loop as the outer periphery of the product, and corrects the coordinates of the end point when so that the final machining segment is extended by the radius of a tool and then passes the resulting data to a machining machine control data generation processing part 4. This processing part 4 converts the outward appearance machining data into machining machine control data required to control the outward appearance machining machine and stores the data in the data storage part 2 again. After all the outward machining data are converted, they are outputted as machining machine control data through a data output part 5 and passed to a trailing process.