To perform servo tracking without reducing a data area by providing a middle layer having a prescribed shaped pattern optically causing contrast between a reflection layer and a back coat layer.

When a beam intensity is zero or smaller than a prescribed threshold value, and is asymmetric to either side of left/right related to a central line of a beam, the beam is made incident on the middle layer 7 also. In such a case, the incident beam is made incident which shifts to either direction of a left side or a right side from the central line of a groove part 8 is judged, and a servo tracking device 24 instructs a servo mechanism 25 and/or a positioning guide roll of a magnetic head unit 21 to move the magnetic head unit 21 to a proper position based on asymmetry of the beam intensity. Thus, the tracking is performed by the servo mechanism 25, and the magnetic head unit 21 returns to the proper position on a data track.