There is disclosed a magnetic disk device of the external synchronization system using a head assembly of the recording/reproducing heads separation type. This magnetic device comprises: a reproducing head for reproducing data from a magnetic disk; a clock signal generator for generating a clock signal on the basis of a reproduced signal corresponding to a clock pattern from the reproducing head; a delay circuit for delaying recording data relative to the clock signal generated at the clock signal generator; a recording head for recording the recording data delayed by the delay circuit onto the magnetic disk; a position detecting circuit for detecting head position information (e.g., track number) in a disk radial direction of the recording head on the basis of the reproduced signal from the reproducing head; and a control circuit for controlling a quantity of delay of the delay circuit on the basis of head position information from the position detecting circuit and a predetermined magnetization reversal pattern.