A numerical control unit for changing a cutting condition of a machine tool during machining of a workpiece. The numerical control unit includes a knowledge storage section for storing at least two rules for changing the cutting condition. At least one of the rules defines a rate at which movement of the machine tool is decelerated based on the position of the machine tool with respect to a corner of the workpiece. Other rules may define the rate at which movement of the machine tool is decelerated based on the shape of the corner of the workpiece being machined. Also, the other rules could be established based on the material of the tool or workpiece. The knowledge storage section further includes a rule description part and assessment functions. The numerical control unit further includes an inferring section for inferring the optimum value of the cutting condition on the basis of at least the rule or rules which define the rate at which movement of the machine tool is decelerated based on the position of the machine tool with respect to the corner of the workpiece. The numerical control unit then causes a workpiece to be machined in accordance with the cutting condition inferred by the inferring section.