A clamp for a disk rotation driving device has a centering hole which is fitted to an outer circumference of a centering projection of a hub of the disk rotation driving device. A clamping screw is screwed into an axial center of the hub whereby a fastening head of the clamping screw fastens the clamp to the hub and a contact portion of the clamp comes in contact with an information recordable magnetic disk attached to the hub and presses onto the disk in a rotation axis direction. The clamp includes a centering cylinder bent in an axial direction. The centering cylinder has a circumferential inner surface which defines the centering hole. The rounded section is provided along an edge of the centering hole and has a radius smaller than the thickness of the centering cylinder. The fastening surface is continuous to the rounded section.