Abstract: A seek command including a cylinder address and a head address is generated from a disk control unit. In a magnetic disk unit, an off-track amount is read out by the head address obtained by the reception of the seek command with reference to a correction table, and an off-track correcting state in which the off-track amount is eliminated from a position control signal is produced during the seeking operation until the start of the position control. A function to instruct the dependent magnetic disk unit to execute the measuring process of the off-track amount in accordance with a time schedule stored in a time table is provided for a higher-order magnetic disk control unit. The generation of a measuring command during the execution of a command chain is suppressed. When receiving the measuring command from the disk control unit, the magnetic disk unit measures the off-track amount of each data disk for a reference position of a servo disk and stores into the correction table.