To provide a disk drive system provided with an inertial latching mechanism which has high reliability and can be arranged in a thin-model disk drive system. There is provided an inertial arm (7) engaging an actuator (22) when a head arm (25) is in or near a parking position and releasing engagement 3with the actuator (22) when the head arm (25) is in or near a position close to a disk (1) and energizing means for holding a position of the inertial arm (7) in a position where the engagement with the actuator (22) is released. Even if a rotational shock is externally applied when the actuator (22) remains in the parking position, moment of rotation is applied on each of the actuator (22) and inertial arm (7) in the same direction, the actuator (22) and inertial arm (7) mutually restrain their operations at an engaging part, and the oscillation of the actuator (22) to a data area can be avoided. When loading, a position of the inertial arm (7) is held by the energizing means in the position where the engagement with the actuator (22) is released so that the release of the engagement and mode of operation of the engagement between the actuator (22) and inertial arm (7) when loading/unloading can be surely achieved.