Embodiments of the invention reduce the effect of the rotational jitters to write a pattern in more accurate timing in self-writing a pattern on a magnetic disk. In an embodiment of the invention, respective patterns in N sectors are first written on an inner circumferential side track as reference patterns with a default clock frequency and a default clock number (a writing interval clock number between respective patterns). The respective reference patterns are then read out plural times for measuring intervals thereof to determine an average value of intervals between the respective sectors. Using the value, a target value is calculated in each sector, which is used in wiring new patterns on the outer circumferential side at even intervals. Then, the respective reference patterns on the inner circumferential side track are read out while the new patterns are written on the outer circumferential side. The respective outer circumferential side patterns are written with the modulated clock frequency after a delay time calculated for every sector has passed from the time when the reference patterns on the inner circumferential side track are detected.