A clamp 1 for a disk rotation driving device 3 has a centering hole 18 and a contact portion 17. The centering hole 18 is fitted to an outer circumference of a centering projection 19 of a hub 7 of the disk rotation driving device 3. A clamping screw 21 is screwed into an axial center of the hub 7 so that a fastening head 23 of the clamping screw 21 fastens the clamp 1 to the hub 7 and so that the contact portion 17 of the clamp 1 comes in contact with an information recordable magnetic disk 5 attached to the hub 7 and applies pressing load onto the disk 5 in a rotation axis direction. The clamp 1 includes a centering cylinder 13 bent in an axial direction. The centering cylinder 13 has a circumferential inner surface. The inner surface defines the centering hole 18. The rounded section 29 is provided along an edge of the centering hole 18 and has a radius smaller than the thickness of the centering cylinder 13. The fastening surface 25 is continuous to the rounded section 29.