PURPOSE: To determine ultrasonic non-linear parameter distribution by easily detecting the phase modulation of measuring beam, by using a pumping pulse wave which is in coaxial relation to measuring beam and advances to a direction opposite to that of said beam.

CONSTITUTION: Ultrasonic beam propagating through a medium 5 to be measured is well subjected to phase modulation by the pumping pulse wave, which is in coaxial relation to the ultrasonic beam and advances to a direction opposite to that of said beam, from a pumping pulse wave generating vibrator 11 without enlarging the vibrator 11 because of no use of a pumping pulse wave in a direction at right angles to the beam axis. This ultrasonic non-linear parameter and phase modulation relating to pumping wave sound pressure are easily demodulated and detected by a receiving vibrator 6 and a position detector 8 and ultrasonic non-linear parameter distribution is simply and rapidly determined as a time function through a function filter 9 without performing Fourier transform.