p4 1 the positional bits p1 2 p2 2 p3 2 and p4 2 the positional bits p1 3 p2 3 p3 3 and p4 3 and the positional bits p1 4 p2 4 p3 4 and p4 4 are in the first servo byte the second servo byte the third servo byte and the fourth servo byte respectively as described above the positional signals are written on the servo disk by shifting the positional bits by the definite time every servo byte successively when this servo disk is used the electromagnetic noise does not superposed on the positional bit in three of the postional bits of the servo byte even though the noise is accidentally superposed on the one of the positional bits in one servo byte in an actual case the positional bits are shifted symmetrically with respect to the regular position l of the positional bit as shown in fig4 fig4 shows the symmetric shift of a positional bit in this embodiment taking the positional bit p1 in fig3 as an example p1 1 p1 2 p1 3 and p1 4 are shifted 30 ns 10 ns 10 ns and 30 ns from the regular position l respectively in fig4 the limit of tolerance of a gate length or gate time for demodulating of the positional bit.