A nondestructive testing apparatus is provided with a wave transmitter by a metal-based magnetostrictive vibrator, a magnetically excited current feeding device for feeding the magnetically excited current to the magnetostrictive vibrator, a wave receiver for detecting an acoustic elastic wave propagating through the measurement object, a filter for extracting a signal in a target frequency band to be measured, and an automatically amplifying rate controlling function-equipped amplifier for automatically controlling the amplifying rate so as to obtain a given magnitude amplitude regardless of the magnitude of the reflection wave or the transmission wave detected by the wave receiver, which constitute a feedback loop. The apparatus is further provided with a signal processor for processing the signal detected by the above-described wave receiver, and a display device for displaying the process result obtained by the signal waveform detected by the above-described wave receiver or the above-described signal processor.