PURPOSE: To estimate the size of a defect, by a method wherein by frequency- analyzing an ultrasonic echo, the defect echo is extracted from the pattern thus obtained, and the image of said extracted echo is displayed on a section displaying apparatus or ultrasonic holography apparatus.

CONSTITUTION: The defect discrimiating circuit comprises a frequency analyzing circuit 41, a characteristic extracting circuit 42, a characteristic comparing circuit 43 and a data storing circuit 44. The circuit 41 frequency-analyzes the ultrasonic echo r larger than a given threshold value as shown by the formula I and calculates the frequency spectral intensity q(f). The circuit 42 receives the data (fi, qi)(i= 1WN) from the circuit 41 and obtains a maximum spectral intensity qm, central frequency f0, average value Δf of the frequency intervals between maximum values, and standard deviation δf. In the circuit 44, the data on the material of the bodies to be inspected and the change according to the difference among reflecting bodies have been registered. The circuit 43 compares the data newly obtained by flaw detection and the data in the circuit 44 with each other to decide whether the reflecting body is defective or not.