PURPOSE: To improve S/N without damaging a real time, by using an ultrasonic pulse of an n-th resonance mode other than the fundamental resonance mode of an ultrasonic transducer and properly selecting output at every vibration to superpose the same.

CONSTITUTION: Driving signals having frequency characteristics of various resonance modes from ultrasonic drive circuits 1 (a, b, c) are changed over by a change-over switch 8 and ultrasonic transducers 6 are excited almost simultaneously in succession (so as to slightly shift a time in practice) to send pulses to an object to be inspected. In the next step, because the signal from the object 7 to be inspected is a mixture of vibration modes f1, f2, f3, it is separated into vibration modes by band zone pass filters (A, B, C). Subsequently, these signals are corrected by the delay circuit in a synthesizing apparatus 5 and added in a synthesizer circuit to enhance S/N. Therefore, by discriminating ultrasonic signals at every vibrations to superpose the same, the transmission and reception of ultrasonic signals are enabled in a wide band zone with good efficiency without damaging a real time and S/N can be made near to a theoretical value.