PURPOSE: To eliminate the fluctuation in a detection level for a defect owing to the mis-alignment between the rotating axis of a probe and a pipe axis by determining and storing said detection level as the function of an angle along the circumference of the pipe by using a reference test piece, using said level in the stage of flaw detection and correcting the sensitivity of flaw detection in real time.

CONSTITUTION: The position of the artificial flaw of the reference test piece is preliminarily detected at every prescribed angle by a sensor 7 for detecting the rotating angle of the rotating part 6 of the probe 2 prior to flaw detection and the detection level of said defect is determined as the function of the angle and is stored in a DAC control device 8. The signal of the probe 2 is inputted via pre- and logarithmic amplifiers 11, 12 to a distance-amplitude correcting circuit 9 and the flaw detection sensitivity is corrected in real time by using the function stored in the device 8 in the stage of flaw detection. The signal is separated to inside and outside defect signals ID and OD by a gate circuit 13.