However, we seem to have a lack of understanding of relations between device parameters and power conversion efficiency, and we hence have not yet reached the full potential of vibration power generators. Furthermore, the acceleration of environmental vibrations observed on infrastructures is very small. For example, a RMS value we measured at a viaduct for a highway was about 0.03 GRMS, and therefore we need to develop a vibration power generator that enables us to charge sufficient electric energy to operate a sensor network device from such a small vibration source. To our best knowledge, there are few papers that report electrically charged energy retrieved from environmental vibrations.