Electret-based vibrational energy generators (E-VEGs) have attracted considerable attention because they can generate electrical power from ambient vibration. E-VEGs have a capacitor structure in which an electret and air gap are sandwiched by electrodes, and the devices are automatically charged by the electric field of the electret. Although various charging processes for dielectric materials to be polarized have been proposed, they lead to low productivity of the device. Recently, we have developed a self-assembled electret (SAE)-based VEG that does not require any charging process. The SAE was realized by utilizing the spontaneous orientation of polar molecules for organic light-emitting diodes: positive and negative polarization charges exist on the film surface and reverse side. Because an electric field will not be formed outside of the SAE, the driving force inducing charge carriers on the electrodes of the SAE-VEG has not been clarified. To clarify the operation mechanism of the SAE-VEG, in this study, the relationship between the surface potential of the SAE and the current generated by electrode vibration in the SAE-VEG is carefully examined. The surface potential is changed over a wide range from negative to positive by controlling the molecular orientation. In addition to establishing a model for the device operation, we demonstrated that the output power of the SAE-VEG can be easily enhanced simply by increasing the thickness of the SAE.