INFORMATION RECORDING DISK

PROBLEM TO BE SOLVED: To ease the allowable width of the flatness of the region to be clamped at the central part of a hard disk by forming parts in contact with upper and lower clamp members in the specified shapes. SOLUTION: A hard disk 1, wherein a signal recording layer is formed on the main surface part of a disk-shaped substrate, has a circular positioning hole 2 at the central part. The main surface parts on both upper and rear sides in the vicinity of the peripheral part are made to be the parts to be clamped. A lower clamp member 6, which is a part of the spindle-motor rotor of a hard-disk drive has a protruding part 6, which is coupled with a positioning hole 2 at the central part of the upper surface. The protruding part is coupled with a recess part 7, which is formed at the central part of the lower surface of an upper clamp member 4. Thus, the parts to be clamped of the hard disk 1 are held from both upper and rear sides. At this time, respective three conical contact projecting parts 3 are formed on both surfaces of the parts to be clamped. These parts are arranged at the corresponding positions at the upper and lower surfaces at the positions having the central angle of 120degrees. The upper and lower clamp members 4 and 5 are approximately in point contact and held the parts to be clamped of the hard disk 1.