PURPOSE: To make the detecting accuracy of loose parts high and to make it possible to record the waveforms when abnormality occurs, by performing filtering by which only a specified frequency band among the vibrating sounds detected by sensors is passed.

CONSTITUTION: Sounds are detected by sensors 1W10. The difference between ordinary background noises and impact sounds is discriminated by loose parts detector 11W20, which are connected to the sensors, respectively. When the presence of the impact sound is detected, the alarm signal for the occurrence of the loose parts is issued. Meanwhile, the level difference is not always clear. Therefore, the background noise level is reduced by utilizing a frequency filter, and the difference between the level and the impact sound is made clear. Based on said filtering, the detecting accuracy of the impact sound in the loose parts detectors 11W20 is enhanced. When the alarm signal is issued from the loose part detectors 11W20, said alarm signal is inputted to a loose parts locator 21, and the true or false of the alarm is judged.