To measure a nonlinear distortion generated in an ultrasonic wave transmitted into a measurement object, without elevating the acoustic pressure.

Two types of input acoustic waves are transmitted by time division from a wave transmitter 14 to the measurement object. A transmission unit 10 multiplies a cyclic waveform to which predetermined nonlinear distortions different from one another have been given, by a window function to make the cyclic waveform into a pulsed input acoustic wave, and for each of the types, transmits the acoustic wave for a plurality of times while changing its phase. A signal processing unit 18 generates a phase sequence by sampling the values of the reception signals of each type of the input acoustic waves at the same time. The signal processing unit 18 performs a linear operation between the both input acoustic wave phase sequences maintained when the measurement object is a linear system, on two types of output acoustic waves and then compares both output acoustic waves to each other. According to the difference waveform, the intensity of a new nonlinear distortion generated in the measurement object is measured.