A disk writer writing servo data on a disk for an HDD. The disk writer includes a reference signal writer writing a reference signal on the disk along a track at a predetermined distance from an edge of the disk; and a servo writer including a reference signal detector detecting the reference signal written on the disk and a writer writing the servo data on the disk on the basis of a position where the reference signal is detected. With this configuration, the present invention provides a disk writer and a writing method thereof, capable of minimizing a positioning error of a start track of an HDD.