Patients who have operation for artificial valve substitution frequently suffer from brain embolism. Micro air bubbles are generated with opening and closing of an artificial valve in such a patient's blood. It is difficult to discriminate air bubbles from emboli by methods using ultrasonic wave. Hence, we tried to develop a new method for discriminating embolus from air bubble in pulsatile blood flow using ultrasound. We utilized the difference in motion of embolus and an air bubble when they are irradiated by ultrasound beam. It is considered that the difference in motion is caused from the difference in acoustic impedance and mass of them. As a result, we succeeded to discriminate emboli from micro air bubbles in pulsatile flows as well as in steady flow in the model experiments in vitro.