Abstract: An apparatus for detecting damage to the cutting tool fitted in a machine tool controlled by automatic machining control circuit means such as a microprocessor, which acts to sense the level of acoustic emission generated by the machine tool during machining, compares that level with one of a plurality of different predetermined data values which are stored in a memory circuit, and generates a warning signal if the noise level exceeds the predetermined level. The different data values stored in the memory circuit are respectively utilized for different machining conditions, i.e. different combinations of cutting tool, machining material, feed rate, etc.