A method includes: performing a time-frequency analysis on measurement data to obtain waveform data representing a temporal change in the intensity of each of a plurality of frequency components; dividing the waveform data of each of a plurality of predetermined frequencies into a plurality of segments so that each section where positive values successively occur and each section where negative values successively occur in a time-axis direction are defined as one segment; calculating the area of each of the segments to obtain segment values; creating, for the waveform data of each of the predetermined frequency components, a selected segment group by excluding a segment whose segment value exceeds a predetermined reference value from the segments in the waveform data; and determining a noise level of each of the predetermined frequency components based on the average value of the segment values of the segments included in the selected segment group.
