Embodiments of the invention provide a magnetic disk unit in which there has been adopted a structure fixing a base and a top plate and preventing a deformation by a bimetal effect. In one embodiment, the magnetic disk unit includes a magnetic disk; a head drive mechanism in which is mounted a head accessing the magnetic disk; a base accommodating the magnetic disk and the head drive mechanism, and possessing a flat part, in which a threaded hole has been formed, in a circumference of an opening edge; and a top cover which is formed by a material whose thermal expansion coefficient differs from that of the base, in which there is provided an escape hole, for a screw, position-aligned to the threaded hole of the flat part, and which is attached to the base by the screw penetrating through the threaded hole and the escape hole for the screw. A slip mechanism for reducing a slip friction resistance is provided respectively in a contact portion between the flat part and the top cover, and a contact portion between a head part of the screw and the top cover.