Embodiments of the invention suppress head jump at the time of application of an impact to provide for an excellent impact resistance performance in a magnetic disk device. In one embodiment, an actuator of a magnetic disk device comprises a suspension mounting the slider on a tip end side thereof to give a predetermined load to the slider, and an arm that holds the suspension, the arm being supported to be able to turn. The arm comprises two arm portions juxtaposed in a direction perpendicular to a surface of the disk, and a spacing between the two arm portions on a side of supported portions thereof in the direction perpendicular to the surface of the disk is larger than that on a side of the slider in the direction perpendicular to the surface of the disk.