To highly accurately detect an off-track amount from an operation result and realize highly accurate control for positioning of a head even when a linearity is not secured in a relationship of the operation result of a positional error using servo burst data read out from a read head and the off-track amount.

A CPU 11 calculates an off-track relationship information of a relationship of an absolute position of a read head 3 within an area of a servo track on a disk 1 and an operation result of a positional error to the absolute value, and stores the information in an EEPROM 16. When the read head 3 is to be positioned to a target track at the time of normal access to data, the CPU 11 can estimate correctly an off-track position of the read head from an approximate curve indicating a relationship value of an off-track amount and the operation result as the off-track relationship information stored in the EEPROM 16.