For the noninvasive diagnosis of heart disease based on the acoustic characteristics of the heart muscle, it is necessary transcutaneously to measure small velocity signals, including components with an amplitude of less than 100 (mu)m/s, and to measure them continuously for periods of more than several heartbeats in a wide frequency range up to 1 kHz. Such measurement, however, has not been realized by any ultrasonic diagnostic method to date. By introducing the constraint least-squares approach, this paper proposes a new method for accurately tracking the movement of the heart wall based on both the phase and magnitude of the demodulated signal to determine the instantaneous position of the object so that the velocity of the moving object can be accurately estimated. By this method, a small velocity of the heart wall with small amplitudes less than 100 (mu)m/s on the motion resulting from a heartbeat with large amplitude of 10 mm can be successfully detected, with sufficient reproducibility in the frequency range up to several hundred Hertz, continuously for periods of about ten heartbeats. The resultant small-velocity signal is analyzed not only in the time domain but also in the frequency domain. The new method offers potential for research in acoustical diagnosis of heart disease.