An ultrasonic diagnostic device for measuring the velocity of blood flow within a living body comprises a receiver for amplifying a received wave in accordance with a logarithmic amplification characteristic and at least one inverse logarithmic converter which produces an output having an inverse logarithmic charac- teristicwith respect to the input thereto. As a result, the device can carry out high-precision measurement of blood flow even when the component of the received wave corresponding to the blood flow is much smaller than a component of the received wave corresponding to the wall of the blood vessel or heart through which the blood is flowing.