PURPOSE: To detect even a slight abnormality of the operation state, by comparing continuously the storage value of a storage device with a new result of the frequency analysis due to an analyzing device in every frequency component.

CONSTITUTION: Detection signals of shake detectors 5 attached to bearings 4 supporting end parts of a rotation shaft 3 are amplified in preamplifiers 7 and are sent to signal converters 8 and are converted to signals suitable for frequency analysis and are inputted to a signal switch 9. If the signal switch 9 is switched to the automatic monitor mode, signals of two signal converters 8 are sent to a frequency analyzing device 10 at different timings at intervals of a designated monitor time. If the signal switch 9 is switched to the manual monitor mode, these signals are sent to the frequency analyzing device 10 after a time required for manual monitor is elapsed simultaneously with switching. These signals are subjected to frequency analysis in the frequency analyzing device 10, and results are stored in a device 11 and are compared with values which are obtained after the lapse of a prescribed time.