F=.SIGMA.(D(t)e.sup.i2.pi.kt/T) (4) where t is a time, k is the wave number, D(t) is data at a time t, T is a section length that is subjected to Fourier transform,and F is the Fourier transform of D(t).As shown in the above-mentioned expression, data to be analyzed is multiplied by a trigonometric function value such as sin or cos which is obtained according to the time t and the wave number k, a