A diagnostic method for detecting rubbing in a slide bearing comprising: detecting waveform data which represents an acceleration of a vibration which occurs when a slide bearing is in operation; detecting a rotational speed of a rotating shaft supported by the slide bearing; transforming acceleration waveform data into an acceleration spectrum of a frequency domain by applying a Fourier transform to the acceleration waveform data; characterized by: data of a frequency domain of at least 1kHz to 30kHz is detected as data which represents the vibration acceleration; quantifying a plurality of peak information in the acceleration spectrum which occurs at the rotational frequency interval of the shaft; wherein the step of quantifying comprises performing a cepstrum calculation which applies an inverse Fourier transform after a logarithmic transformation to the acceleration spectrum; obtaining time series data of a quefrency value obtained from the waveform data after the cepstrum calculation; monitoring whether the quefrency value has exceeded a predetermined threshold value; and when the quefrency value has exceeded the threshold value, determining that an abnormality has occurred in the slide bearing. detecting waveform data which represents an acceleration of a vibration which occurs when a slide bearing is in operation; detecting a rotational speed of a rotating shaft supported by the slide bearing; transforming acceleration waveform data into an acceleration spectrum of a frequency domain by applying a Fourier transform to the acceleration waveform data; characterized by: data of a frequency domain of at least 1kHz to 30kHz is detected as data which represents the vibration acceleration; quantifying a plurality of peak information in the acceleration spectrum which occurs at the rotational frequency interval of the shaft; wherein the step of quantifying comprises performing a cepstrum calculation which applies an inverse Fourier transform after a logarithmic transformation to the acceleration spectrum; obtaining time series data of a quefrency value obtained from the waveform data after the cepstrum calculation; monitoring whether the quefrency value has exceeded a predetermined threshold value; and when the quefrency value has exceeded the threshold value, determining that an abnormality has occurred in the slide bearing. data of a frequency domain of at least 1kHz to 30kHz is detected as data which represents the vibration acceleration; quantifying a plurality of peak information in the acceleration spectrum which occurs at the rotational frequency interval of the shaft; wherein the step of quantifying comprises performing a cepstrum calculation which applies an inverse Fourier transform after a logarithmic transformation to the acceleration spectrum; obtaining time series data of a quefrency value obtained from the waveform data after the cepstrum calculation; monitoring whether the quefrency value has exceeded a predetermined threshold value; and when the quefrency value has exceeded the threshold value, determining that an abnormality has occurred in the slide bearing. performing a cepstrum calculation which applies an inverse Fourier transform after a logarithmic transformation to the acceleration spectrum; obtaining time series data of a quefrency value obtained from the waveform data after the cepstrum calculation; monitoring whether the quefrency value has exceeded a predetermined threshold value; and when the quefrency value has exceeded the threshold value, determining that an abnormality has occurred in the slide bearing.