An inertial sensor for the vehicle sensitive triggering of a locking mechanism on a belt retractor comprises a ball which is seated in a socket in alignment with a reference plane. A pivotally mounted sensor lever rests on the ball and has a free end with a control pawl formed thereon. The sensor lever is formed and adapted for cooperation with an externally toothed control disk of the belt retractor. The pivot axis of the sensor lever is inclined in relation to the reference plane in the same direction as the plane of the control disk, the angle of inclination of the pivot axis of the sensor lever being approximately half as large as the angle of inclination of the plane of the control disk in relation to a plane perpendicular to the reference plane.