PURPOSE: To analyze a vibration spectrum by controlling the resolution of a spectrum analysis and controlling a pass band automatically so that the interference distortion of the result of vibration spectrum analysis can be eliminated.

CONSTITUTION: A vibration signal that is picked up from a rotary machine by a vibration pick-up 1 reaches the input terminal of a low-pass filter 2. The pass band of the filter 2 is shaped by activation sections 121-12K and feedback constituents 131-13K. Also, the filter 2 limits the upper-limit frequency of the signal spectrum. In this case, a signal is sent to the input of an A/D converter 3, and amplitude and time are digitized at a sampling speed that is proportional to the rotary speed of a machine in the converter 3. Then, the rotary speed is detected by a rotary speed sensor 8 and is applied to the input of a frequency multiplication equipment 7, thus obtaining a means for changing the pass band of the filter 2 automatically to eliminate the spectrum distortion due to the overlapping of the generated interference and hence analyzing vibration spectrum.