Provided is a technique forming a highly stable inertial sensor with a simple configuration by vacuum sealing a resonator which detects acceleration and exploiting a resonance vibration using a high Q value of a MEMS device. The inertial sensor includes: a detecting proof mass and beam 105 which detects acceleration; a driving electrode 106 which excites the detecting proof mass and beam 105; a resonant frequency tuning electrode 108 which changes the resonant frequency of the detecting proof mass and beam 105; and a detecting circuit which applies voltage (frequency controlling part 433, DAC part 436) to the resonant frequency tuning electrode 108 for changing the resonant frequency to cancel a change of the resonant frequency of the detecting proof mass and beam 105 when the acceleration is applied to the detecting proof mass and beam 105 during the vibration of the detecting proof mass and beam 105 by the voltage applied to the detecting proof mass and beam 105, and outputs the acceleration (CV converting part 430, ADC part 431, demodulation part 432, frequency controlling part 433, signal adjusting part 435) based on a value of the voltage applied to resonant frequency tuning electrode 108.