PURPOSE: To make automatic operation possible at all times under maximum load that would not generate abnormal vibrations, by incorporating, into various instruments the same apparatus for detecting the generation of vibrations that allows to detect the suppression of the vibrations.

CONSTITUTION: The block diagram illustrates the case where cutting is effected by a lathe. For example, strong synchronous constituents are contained in detected signals from a bit even before abnormal vibrations have occurred, and these detected signal parts are suitably selected in accordance with the application. These detected signals are amplified, and they are judged through a band pulse filter, an A-D converter, a CPU, etc. whether there is synchronism of the peaks of the signals. When the synchronism is detected N times, the CPU outputs a load lowering signal to a lathe control section to decrease the depth of the cutting. By this suppression of abnormal vibrations, the CPU sends to the lathe control section a signal for stopping the operation to decreased the load and that state is kept to continue the cutting.