PURPOSE: To prevent the deformation of a disk and spindle and to prevent the generation of swarf by providing three spindle projecting parts which can be detained to disk detaining parts respectively at 120° pitch in the inner peripheral and outer peripheral parts of the disk.

CONSTITUTION: A hole 22 to be fitted onto the spindle 20 is formed in the central part of the disk 21. The 1st to 3rd disk detaining parts 23 to 25 having the same shape with which the distance L from the center of rotation of the disk 21 decreases gradually toward the rotating direction (arrow direction) of the spindle are formed at 120° pitch in the inner peripheral part of the disk 21. On the other hand, the 1st to 3rd spindle projecting parts 26 to 28 with which the distance L' from the center of rotation of the spindle 20 is smaller than L1 and larger than L2 and which can be respectively pressed to the 1st to 3rd disk detaining parts 23 to 25 are formed at 120° pitch on the outer peripheral surface of the spindle 20. As a result, the disk 21 is restrained and, therefore, the offcentering of the disk 20 does not arise. The generation of the swarf is prevented as well.