A magnetic disk apparatus capable of reducing deformation of a magnetic disk is disclosed. The magnetic disk apparatus comprises a magnetic disk on which data is recorded while the disk is driven, a hub around which the magnetic disk is supported from an under side of the magnetic disk, and a clamp in contact with the other side of the magnetic disk and fixing the magnetic disk. An O-ring is interposed between a flange formed at an outer circumference of the hub and the magnetic disk, and a spacer is interposed between the magnetic disk and the O-ring. Thus an upper surface of the hub is in contact with an under surface of the clamp and defines a vertical position of the magnetic disk.