PURPOSE: To increase the accuracy of analysis, by obtaining the attenuation factor of the reponse amount after obtaining the amplitude through the Fourier trnasformation and in the finite section decided with the cycle of the natural vibration mode for the optional one of the several natural vibration mode components which are sampled in a certain time interval and then repeating the amplitude obtained with a time shift equivalent to every fixed number of data.

CONSTITUTION: The Fourier transformation gives a linear spectrum in terms of the time-series digital data in the finite section sampled in a certain time when the input function is the cycle function which becomes an integer cycle in the finite section. Based on this throry and for an optional mode among the time-series digital response data containing the plural natural frequency components, a Fourier conversion is carried out in the section T=N/f0 (N=1, 2...) which is decided by the frequency f0 of obtain the amplitude. The amplitude is obtained by selecting a small value for the N and also giving a time shift gradually to the data sections, thus obtaining the attenuation factor of the mode component each. In such way, both the response amount and the attenuation factor can be obtained in a high accuracy.