PURPOSE: To set a servo signal for each individual disk of plural sets of drives respectively by controlling the position of a head of each drive and setting the servo signal by means of a servo writer having a laser length measuring machine common to the plural sets of drives.

CONSTITUTION: Setting the servo signal to the drives 1A and 1C respectively is completed, and then the setting is performed to the drive 1B. Since a laser beam LT to a mirror 533B of the drive 1B is intercepted by a mirror 533A of the drive 1A, the mirror 533A is turned to be withdrawn, and the servo signal is set to the drive 1B. That is, a proper bias current is supplied to a voice coil of the drive 1B by a carriage control circuit to stop the head 12B in the vicinity of the center of the disk 2B. On the other hand, a head arm is moved by supplying a driving current from a head arm control circuit 54. At the same time, a moving amt. of the moved mirror 533B is measured by the laser length measuring machine 531, and is checked by an MPU 34, and then the position of the head 12B is controlled by the control circuit 54, and the servo signal is written.