To suppress pressure fluctuation caused by insertion of arms between disks, which arms support thereon magnetic heads, to reduce disk flutter to thereby reduce disk driving power in a magnetic disk apparatus, there are provided an open space not covered by a shroud, which constitutes a part of a housing structure to cover a side surface of a disk stack, and provided on a downstream side of the arms relative to an airflow moving along surfaces of the disks as the disks rotate, and a channel, which connects a space on an upstream side of the arms and the open space not restricted by the shroud on the downstream side of the arms, the channel being provided with a linear section, which has a rectifying effect on the airflow.