PURPOSE: To simplify correcting operations in correcting a dimension error of a cutting tip, by determining the correction value from a measured value, generating pulses corresponding to the correction value using a manual pulse generator, and inputting them in an numerical control device.

CONSTITUTION: Pushing a start button, a tool rest 6 receives a tool from a magazine 10 and returns to the standby position. Then a tool 5 is moved in the X direction, and changeover is made to cutting feed in a certain position. When the tool 5 reaches the cutting start point, some trial cutting is made, and a return button 28 installed at a correction value determining unit 22 is pushed to move the tool 5 to an appropriate retracted position, followed by stopping. Then the deviation from the set value is measured using a measuring piece 21, and the result is fed into the correction value determining unit 22 in the form of electrical signals to serve for determination of the correction value. The pulses corresponding to this correction value are generated by a pulse generator 25 and fed into a correction value input circuit 27. This circuit 27 corrects the operation circuit by means of entry into the operation circuit of a numerical control device 23. Then the tool 5 is restituted to proceed to the second trial cutting, followed by normal run.