A magnetic disk apparatus arranged in a housing having a plurality of magnetic disks stacked on a rotating shaft, arms supporting thereon magnetic heads and adapted to be inserted between said magnetic disks, a rotary actuator for moving the arms, a housing forming at least a part of an arcuate-shaped shroud which substantially surrounds a portion of side surfaces of the magnetic disks so as to permit movement of the arms and which is concentric with the magnetic disks so as to define a gap between the shroud and the portion of the side surfaces of said magnetic disks. The gap defined between the outer peripheral end surfaces of the magnetic disks and the shroud is in a range of 0.1 mm to 0.6 mm.