To provide a detecting method for fluctuating frequency of signals which permits frequency component of signal fluctuation to be detected for a short detecting time with high response, while keeping necessary resolution for frequency.

The data is sampled with a prescribed period and converted digitally (10), and stored successively by a number of power of two to generate a data group. Whenever a data group is generated, the data group is put in time order by a number of power of two to generate the first data line (11) and it is analyzed with the fast Fourier transform to obtain the spectrum of the first fluctuating frequency (12). In the first data line, all the data in data groups other than the latest data group are permuted to zero to generate the second data line (15), and it is analyzed with the fast Fourier transform to obtain the spectrum of the second fluctuating frequency (16). Then the third fluctuating frequency spectrum is obtained through multiplication or geometrical mean of each oscillation of the first and the second fluctuating spectra at the same frequency (14, 14a), to gain the peak frequency of the third fluctuating frequency spectrum.