PURPOSE: To eliminate the effect of a varying noise and to make the detection easy by finding the differential output signal between the acceleration sensor having a resonance frequency within the specified frequency range and the acceleration sensor having a uniform sensibility characteristic for the vibration of less than the specific frequency and without resonance frequency within the same frequency range and by comparing with the output signal of a rotary detecting sensor.

CONSTITUTION: The 1st acceleration sensor 11 having a resonance frequency within a frequency range of 100W200kHz and the 2nd acceleration sensor 12 having no resonance frequency in the same frequency range and having a uniform sensibility characteristic for the frequency of less than 20kHz are fixed to the almost same place of a rotary machine 16. The output signals of the sensors 11, 12 become detecting signals 6a, 15a, via amplifiers 4, 13, filters 5, 14 and detectors 6, 15. Since a rubbing vibration and varying noise are caused at the fitting place of the sensors 11, 12, the signals 6a, 15a are inputted into a differential amplifier 16 an the differential signal 16a of only rubbing signal from which the varying noise is removed is obtd. The signal 16a is then compared with the output signal of the rotary detecting sensor 3 and the rubbing is detected without the effect of varying noise.