According to one embodiment, a magnetic disk device includes a disk, a head including a read head that reads data from the disk, a write head that writes data into the disk, and an assist element that generates energy for improving write performance of the write head, and a controller that adjusts a degauss condition after a stop of a write process of the write head, based on a resistance value of the assist element, and adjusts a data format of the disk in accordance with the degauss condition, the resistance value being measured after data is written into the disk, and the write process having an influence on the resistance value.