The subject of the present invention is to highly accurately remove the frequency component of an electromagnetic vibration caused by an inverter power supply from a signal mounted to a vibration sensor of a rotary machine driven by the inverter power supply. For this purpose, Fourier transformation is performed on the vibration time waveform acquired by the vibration sensor, and a frequency spectrum is calculated. The frequency of the maximum wave peak at the periphery of the carrier frequency of the inverter power supply is set to be the reference frequency. A self-correlation function of the frequency spectrum around the reference frequency is calculated. The interval of the wave peaks of the self-correlation function at the periphery of the reference frequency is obtained. The wave peaks existing at intervals of the equal-interval wave peaks existing at the periphery of the reference frequency are extracted as object peaks. The method is used for reducing the level of the frequencycomponent of the object peak in the frequency spectrum.