According to one embodiment, a magnetic disk device, including a disk, a head to write data to the disk and read data from the disk, a preamplifier to generate a recording current corresponding to data that the head writes to the disk, and a controller to convert a first data pattern in first write data, in accordance with a pattern length of a second data pattern previous to the first data pattern, to a different data pattern including a pseudo polarity inversion that does not cause a polarity inversion when converting the first data pattern to the recording current.

According to one embodiment, a magnetic disk device, including a disk, a head to write data to the disk and read data from the disk, a preamplifier to generate a recording current corresponding to data that the head writes to the disk, and a controller to convert a first data pattern in first write data, in accordance with a pattern length of a second data pattern previous to the first data pattern, to a different data pattern including a pseudo polarity inversion that does not cause a polarity inversion when converting the first data pattern to the recording current.