A magnetic disk drive is to be provided which not only can promote the reduction of size thereof but also maintain a state in which outside air does not directly enter the interior of a case. In one embodiment, a magnetic disk drive comprises a base member, the base member having an upper surface at least a part of which is open to receive therein a magnetic disk assembly, a plate-like cover member for covering the magnetic disk assembly received within the base member, and a sheet-like sealing member affixed to an upper surface of the cover member and upper and side faces of the base member to seal a clearance between the base member and the cover member.