The aim of this study was to develop a method for estimating the cross-sectional area function of the vocal tract by acoustic measurement. In this method, the area function is estimated using the transient response determined from the incident and reflected signals measured at the lips. The measurements are made with the vocal tract at one end of uniform tube that is excited at the other end by a speaker. With this method, it is extremely difficult to estimate the area function behind the narrow constriction, because the input and output signals are measured on the same side of the vocal tract. To improve estimation accuracy, a wide range and high energy level of frequency characteristics is needed for the input signal. The method for obtaining the desired input signal introduced here is based on the LMS error of the speaker output. To verify the estimation accuracy, an experiment was carried out using some cavities of known geometry. This measuring system shows good performance for the cavity whose ratio of minimum to maximum area is about 1:20. The estimated area functions of the human vocal tract were compared with midsagittal shapes simultaneously measured by a magnetic position sensing device.