PURPOSE: To judge the presence or not of any flaw of the position to be examined from the correlations of respective receiving acoustic waves by providing a correlator which judges the correlations from the propagation acoustic waves from a piezoelectric pulser.

CONSTITUTION: A piezoelectric pulser 2 is excited by the signal having amplified the output of an oscillator 5 with a power amplifier 6 and generates impulsive acoustic waves. The acoustic waves propagate in the member to be examined 1. Pizoelectric sensors 3, 4 are subsequently arrayed in the propagating direction of said acoustic waves and the acoustic waves are received respectively thereby. The receiving timings thereof vary with the time differences required for propagation of the acoustic wave in the member to be examined 1. And the signal having been received in the first piezoelectric sensor 3 is supplied to one terminal of a correlator 10. The signal having been received in the second piezoelectric sensor 4 is being inputted to the other terminal of the correlator 10.