A magnetic disk device includes a magnetic disk, and a magnetic head slider. The slider has a magnetic head, a plurality of surfaces which are formed sequentially from an adjacent side to a magnetic disk, and a slider rail surface which is arranged near an air flow-out edge on a nearest surface of the plurality of surfaces. The slider rail surface includes a long sideways rail surface on an air flow-in side and a lengthwise rail surface on an air flow-out side. The nearest surface of the plurality of surfaces is arranged only in a center area with respect to a width of the slider near the air flow-out edge and has a width substantially less than the width of the slider. An area of the long sideways rail surface is larger than an area of the lengthwise rail surface.