This paper describes experimental results of polarization charge density and residual charge density on PET-film/plane-electrode contact with a thin air layer by a pulsed electroacoustic method. In this method, the air layer has an important role as an acoustic barrier to separate the charge on the film from the charge on the electrode. When a dc applied voltage exceeded a critical level, the polarization charge density deviated from its theoretical value. The quantity of the deviation had a good agreement with the quantity of increase in the residual charge density after short-circuiting. These results suggested that some true charges were trapped at the surface states of PET by the voltage application and they could not be returned to the electrode by their own internal field. Critical field strength across the air layer for trapping was calculated from the results. True charge density on the electrode surface could be also measured by the use of this method.