PURPOSE:To securely detect a defect even if a body to be measured is long by inputting an acoustic wave to one end of the body to be measured and analyzing transmission characteristics of the acoustic wave exiting from the other part on a frequency dependency frequency axis. CONSTITUTION:A transmitting transducer 2 is fitted to one end of the body 1 to be measured and a receiving transducer 3 is fitted to the other end. Then, a signal oscillator 5 sends a white noise to the transmitting transducer 2 and part of the white noise is branched and inputted to a fast Fourier transforming analyzer (FFT) 4. Then, the acoustic wave propagated in the body 1 to be measured is inputted to the FFT4 from the receiving transducer 3 to make an analytical comparison with the original white noise by the frequency range. At this time, when the white noise is inputted to the body 1 to be measured, the frequency and amplitude of a detection wave have such a relation that the wave attenuates abruptly at a place A of some frequency according to the quantity of the defect, and this point A is considered to be an attenuation completion point. Then, the frequency at the attenuation completion point A becomes lower and lower as the body has more and more defects. Thus, the attenuation completion point A is measured on a frequency axis, so defects in internal organization variation, density variation, etc., are accurately detected.