The deviation of head and track at a most outside position conference track (PtO) and at a most inside position conference tract (Pti), the deviation value is processed in accordance with the linear interpolation, and the deviation of the head from a certain data track can be forecasted. If these position reference tracks (Pt) are used and the deviation of head from the certain track is processed according to the interpolation of curve of second order, the deviation of the head from the track is forecasted more precisely, and the deviation can be compensated for to trace the track center correctly.