We report a SiO_2 electret for comb-drive actuators that have vertical and narrow gaps. The potassium ion included SiO_2 film was formed on the comb-drive actuator by thermal oxidation. Because the SiO_2 film polarized, the bias-temperature (B.T.) treatment was processed to the actuator. Without applying the bias voltage, the resonance peak was observed by inputting a minute AC signal (1 [V]) in the vacuum. This resonance peak is evidence that SiO_2 Electret was formed. This charging method is possible to apply to energy harvesting devices that use comb-drive actuators.