According to one embodiment, a magnetic disk device includes a disk having a first sector including first servo data, first user data, and first correction information used to correct a data error, a head that writes data to the disk and that reads data from the disk, and a controller that changes, according to a first defect length of a defect generated in the first sector in a circumferential direction of the disk, a first sector length of the first sector in the circumferential direction.