PURPOSE: To perform high-precision monitoring by finding the energy distribution of the frequency of an ultrasonic wave propagating in a body to be inspected near the surface, finding the degree of its fatigue from the attenuation in a high frequency range, and also finding the depth of the fatigue from the lower limit frequency of a frequency range where attenuation occurs.

CONSTITUTION: An ultrasonic wave is outgone from a transmitter 2, and propagates in the inflection part of piping 1 to reach a receiver 3, which receivers it. A signal from the receiver 3 is sent to a signal processing circuit 6 through an amplifier 5. An energy distribution measuring circuit 7 finds the energy distribution of the frequency of the received ultrasonic wave, and a time difference measuring circuit 8 measures the time up to the arrival of the ultrasonic wave at the receiver 3. A discriminating circuit 9 compares normal energy distribution with the found energy distribution to find attenuation ΔE at a frequency fM in some band of a high frequency range, calculating the degree of fatigue. Further, a low limit frequency fF in a band where attenuation of energy is caused is found to calculate the depth of the fatigue from the surface.