A servo track writer and a chuck thereof which are capable of maintaining a central axis of a disk hub to be aligned with a rotating axis of a spindle motor adapted to rotate the disk hub, thereby preventing disks mounted on the disk hub from fluctuating. The servo track writer includes a disk hub, on which a plurality of disks are mounted, a magnetic head assembly to write servo information on the disks mounted on the disk hub, a spindle motor to rotate the disk hub, and a chuck to firmly hold the disk hub. The chuck firmly holds the disk hub at a plurality of points to prevent the central axis of the disk hub from being inclined from the rotating axis of the spindle motor.