According to one embodiment, a magnetic disk device includes a plurality of magnetic disks including a first recording surface and a second recording surface, a first magnetic head that writes data to the first recording surface, a second magnetic head that writes data to the second recording surface and a controller that includes a counter configured to detect the number of writes of each of the first magnetic head and the second magnetic head, an unwritten detection unit configured to detect an unwritten block from the blocks, and a memory configured to store a first threshold for the number of writes of each of the first magnetic head and the second magnetic head and a second threshold less than or equal to the first threshold.