PURPOSE: To improve the Doppler sensitivity of a pulse Doppler type ultrasonic Doppler system by automatically changing the reference frequency in response to the sample point depth of the aimed site in a testee.

CONSTITUTION: A reference frequency changing circuit is provided in a Doppler measurement section 3 as a means automatically changing the reference frequency in response to the optional sample point depth given from an operation panel 6. The reference frequency can be automatically changed in response to the sample point depth of the aimed site in a testee, thus the Doppler sensitivity can be improved.