According to one embodiment, a magnetic disk device includes a disk medium which has a first region, a second region, and a redundant third region, and a control circuitry. The first region is a region to which first data received from a host is written. The second region is a region to which the first data is temporarily written before the first data is written to the first region. The control circuitry executes defect detection to the first region. Upon detecting a defect, the control circuitry substitutes a first location having the defect in the first region with a second location in the third region. Depending on a total size of free regions of the third region, the control circuitry reallocates a fourth region from the second region to the third region. The fourth region is part or all of the second region.