PURPOSE: To improve the machining efficiency, to stabilize the machining accuracy, and to prevent a deterioration of tool service life, by generating a numerical control data to make the load in the machining constant regardless of the form of a raw material.

CONSTITUTION: Depending on the tool data, the machining amount data, the machining locus data, and the raw material data input through an input means 6, and the tool position data found by a machining starting position setting means 10, the feed speed of a tool on the way of the machining amount part according to the form of the raw material is decided by a feed speed deciding means 12. The raw material form and the tool position in the following machining are found by a machining halfway form producing means 13, and by repeating the operation to decide the feed speed of the tool by the feed speed deciding means 12, a numerical control data to set the feed speed of the tool making the machining load constant is provided.