a magnetic head including a write head configured to write data on a disk medium and a read head configured to read out the data from the disk medium;

an actuator configured to have the magnetic head mounted thereon;

a memory unit configured to store offset information for, when the magnet head is positioned to a target position over the disk medium, adjusting an offset based on a positional relation between the write head and the read head;

a controller configured to, at a data read/write operation time, drive the actuator under control with the use of the offset information stored in the memory unit and position the write head or read head to the target position over the disk medium;

a measuring unit configured to measure a disk runout amount and phase of the disk medium at an operation time other than the data read/write operation;

an offset information updating unit configured to generate correction offset information according to a result of measurement by the measuring unit and update the offset information of the memory means by the correction offset information;

a unit configured to decide whether or not a result of measurement made by the measurement unit is an abnormal value upon comparison with a reference value; and

a unit configured to, when the result of comparison is an abnormal value, make a notice to that effect.