PURPOSE: To enable formation or a sound field in an arbitrary body to be inspected, which is optimum both for an analysis of the physical properties of the body and for internal flaw detection therefor, by forming a vibrator to be circular and also by changing the frequency of an electric signal to be transmitted from a signal processing means to the vibrator.

CONSTITUTION: An ultrasonic pulser 2 transmits a high-frequency pulse signal to a probe 1 and a vibrator 21 of the probe 1 is excited by this signal and transmits an ultrasonic wave to an ultrasonic wave propagation medium 24. A back sound field wherein the first minimal position corresponding to the position of formation of the circular vibrator 21 is located is formed at the back of an acoustic lens 25, and the first minimal position is shifted by changing the frequency of the high-frequency pulse signal by a frequency converter 3. With the back sound field made to be an optimum sound field in accordance with a purpose of use, an ultrasonic wave is applied from the probe 1 to a body 12 to be inspected, a reflected wave thereof is received and converted into an electric signal by the vibrator 21, and a peak value of the wave is detected by a peak detector 5 and inputted to a microcomputer 7 to be processed. Data obtained by the processing are stored, printed by a printer 8 and displayed on CRT.