A servo pattern is positioned in a DC erased portion to which a DC erase process has been performed, and recording tracks and track groove portions (discrete portions) are positioned in an AC erased portion to which an AC erase process has been performed. The track groove portions extend in a circumferential direction. The AC erased portion and the DC erased portion are both made of a magnetic material. In the servo pattern, four lines of preamble portions extending almost in a radial direction are formed and a plurality of pit portions disposed in a dotted manner in two lines are arranged. The pit portions are positioned rearward of the preamble portions in respect of a scanning direction of a magnetic head.