PURPOSE: To almost eliminate the attenuation of an AE wave by fitting a sensor for detecting AE wave direct detecting the AE wave generated at the tool damaged time to a main shaft.

CONSTITUTION: In either case of working a work W fitted to a main shaft 4 and driven by its rotation by a cutting tool 6 or working the work W by the cutting tool 6 fitted to the main shaft 4 and driven by rotation, AE wave generated at the time when the cutting tool 6 is damaged during cutting is transmitted to the sensor 17 for detecting the AE wave fitted to the main shaft 4 via the work W fitted to the main shaft 4 and the main shaft 4 or via the main shaft 4 only. This AE wave detected electric signal is transmitted in a noncontact state to the secondary side which is nonrotating member 15 side from the primary side which is a rotating main shaft side via the electromagnetic induction action, etc., of between the primary side and secondary side of a noncontact electric bonding means 19, the AE wave detected signal reproduced at the secondary side is processed in a detection signal processing circuit 26 and the automatic stoppage of the main shaft and the operation of an alarm are performed.