PURPOSE: To provide the matching condition for an impedance matching unit provided between a directional coupler and ultrasonic wave probe automatically every time the type of the probe is changed.

CONSTITUTION: An impedance matching device is provided with an oscillating means for sweeping transmitting frequency, a means for detecting the maximum value of reflected signal of a probe 1 obtained from the sweeping transmitting output and storing the transmitting frequency corresponding to the maximum value and an automatic adjusting means for adjusting an adjustable impedance matching unit 7 on the basis of the transmitting frequency to maximize reflected signal value of the probe 1.