In this paper, a method for measuring the acoustic impedance density from sound pressure distribution is presented. Experiments were performed for a plaster replica of the lips together with the oral cavity and for a quasi-elliptical uniform tube with a wedged-shaped end imitating the lip horn. The change of frequency characteristics of the acoustic impedance density along the center-line inside the lips indicates that the labial horn acts as an acoustic wave guide. The theoretical characteristics of the acoustic impedance density derived from an idealized elliptical piston model and a pulsating sphere model are also shown for the better determination of the boundary condition for FEM computation.