An inertial sensor having a simple configuration by vacuum sealing a resonator which detects acceleration and exploits a resonance vibration using a high Q value MEMS device. The sensor includes: a detecting proof mass and beam which detects acceleration; a driving electrode which excites the detecting proof mass and beam; a resonant frequency tuning electrode which changes the resonant frequency of the detecting proof mass and beam; and a detecting circuit which applies voltage to the resonant frequency tuning electrode for changing the resonant frequency to cancel a change of the resonant frequency of the detecting proof mass and beam when the acceleration is applied to the detecting proof mass and beam during the vibration of the detecting proof mass and beam by the voltage applied to the detecting proof mass and beam, and outputs the acceleration based on a value of the voltage applied to resonant frequency tuning electrode.