PURPOSE: To perform orientation of the position of the sound source of a vibration wave propagating on a structure by a method of pattern recognition.

CONSTITUTION: A time delay T at a rise point of the vibration wave propagating from the sound source is measured and a square integral value of each signal amplitude is found. A value obtained by dividing each by the maximum P out of a partial power P obtained in this way is determined as a relative partial power RP. When the time delay T and the relative partial power RP thus obtained are plotted on vertical and horizontal axes respectively, a specific pattern is formed in the position of the sound source. On the surface of the pressure vessel 11 of a nuclear reactor an accelerometer 12 is arranged. A strike is applied to the expected position of the sound source of the pressure vessel of the reactor beforehand, the vibration wave is generated thereby, and a reference pattern corresponding to respective positions is obtained from this vibration wave. The orientation of the position of the sound source is conducted by selecting a reference pattern which is most similar to a pattern formed by the vibration wave generated from the sound source to be orientated.