We have developed a micro-electro-mechanical systems (MEMS) electrostatic vibratory power generator with over 100 $\mathsf{\mu }$ W ${}_{\mathrm{RMS}}$ of (root-mean-square) output electric power under 0.03 G ${}_{\mathrm{RMS}}$ (G: the acceleration of gravity) accelerations. The device is made of a silicon-on-insulator (SOI) wafer and is fabricated by silicon micromachining technology. An electret built-in potential is given to the device by electrothermal polarization in silicon oxide using potassium ions. The force factor, which is defined by a proportional coefficient of the output current with respect to the vibration velocity, is 2.34 × 10 ${}^{-4}$ C/m; this large value allows the developed vibration power generator to have a very high power efficiency of 80.7%. We have also demonstrated a charging experiment by using an environmental acceleration waveform with an average amplitude of about 0.03 G ${}_{\mathrm{RMS}}$ taken at a viaduct of a highway, and we obtained 4.8 mJ of electric energy stored in a 44 $\mathsf{\mu }$ F capacitor in 90 min.