1. A disk drive comprising: a disk having a data area for recording data and a servo area in which servo data is prerecorded; a head for performing a normal read/write operation of reading and writing data in and from the data area and for reading the servo data from the servo area to achieve a head positioning control; actuator means supporting the head and designed to move the head to a desired position on the disk; servo means for calculating, from the servo data read by the head, a position error which the head has with respect to the desired position during the head positioning control, and for calculating, from the position error, a control value for driving the actuator means; learning means for calculating, from the position error, a learned value relating to a disk-runout occurring while the disk is rotating; learning control means for determining whether the learned value deviates from a prescribed tolerance value and for abandoning the learned value when the learned value is erroneous one deviating from the prescribed tolerance value; and servo compensation means for compensating for the control value by using the learned value, when the learned value is an effective one while the servo means is performing the head positioning control during the normal read/write operation.