PURPOSE: To make measurements on the processing part immediately after the processing and detect tool damages, by providing measuring devices on a stage which moves together with the processing tool against the object being processed.

CONSTITUTION: With the progress of processing, measuring devices 12W14, together with processing tool 4, are sent in axial direction 6. During this time, the contact roller is pressed against the surface of object under processing 3 by the coil spring at all times. When the diameter of the processing-completed part increases suddenly due to a damage or any other trouble occured to processing tool 4, the measurements made by the above-described measuring devices 12W14 increases so that they exceed the values set by comparators 51W53, signals are issured from comparators 51W53 to AND circuit 54, and further, such signals enter processing circuit 55, and the relay, which controls the motor circuit of lathe 2, is cut. As a result, the rotation of this motor is stopped and also the operation of lathe 2. In this way, improper processing due to tool damage is prevented.