Disclosed herein is an inertial sensor. The inertial sensor includes a sensing unit and a driving-mass-position initialization module. The sensing unit includes a driving mass, a flexible board unit which displaceably supports the driving mass, and a support which supports the flexible board unit to allow the driving mass to move in a suspended state. The flexible board unit has driving electrodes which move the driving mass, and sensing electrodes which sense the movement of the driving mass. The driving-mass-position initialization module includes a position initialization member which reciprocates to initialize the position of the driving mass, and a coil unit which surrounds the position initialization member. An initialization-member-receiving depression is formed in the driving mass. The shape of the initialization-member-receiving depression corresponds to that of the position initialization member.