PURPOSE: To set each servo track at a correct position by correcting a positional shift of a head at a write time and a read time in a servo track writer whenever the head shifts.

CONSTITUTION: The shifting amount δ of a head H between a write position and a read position to the last one of a plurality of servo tracks e.g. TR4 is measured by a gap sensor Gs. The measured positional shift δ is corrected on the basis of a reference position updated by moving a moving part Ka, so that the head is correctly positioned. Servo tracks TR are sequentially set on the whole surface of a disc D. Accordingly, each servo track is set at a correct position and improved in reliability.