Embodiments of the invention provide a clamping mechanism capable of clamping a magnetic disk by an accurate fastening force. In one embodiment, a magnetic disk fastening method includes the steps of: mounting a magnetic disk on a hub structure, setting a thin plate spring having a pressure-bearing part, a flexible part and a disk holding part at a predetermined position relative to the magnetic disk with the disk holding part in contact with a surface of the magnetic disk; pressing the pressure-bearing part by screwing a fastening screw in a threaded hole formed in the hub structure; measuring a deflection by which the flexible part is deflected in the pressure-bearing part pressing step; and stopping pressing the magnetic disk upon the increase of the deflection to a predetermined value.