The vocal tract shape is measured by MRI, is approximated as a tube with an elliptic cross section, and we evaluate the approximation accuracy in the transfer function by using our FEM analysis. Our computational results show that our approximation method has enough accuracy in comparison with the real cross sectional shape of the vocal tract. We discus the lip impedance and the effect of the higher mode when the driving surface is located on only a partial area, and when the tract has small branches such as the pyriform sinus and the branch related to movement of the epiglottis.