According to an embodiment, a magnetic disk device includes a magnetic disk, a magnetic head that accesses the magnetic disk, a motor that moves the magnetic head, a ramp, a motor driver, and a processor. When a voltage value of a power supply voltage falls below a first threshold, the motor driver starts a retract operation of retracting the magnetic head to the ramp by controlling a voltage applied to the motor. When the voltage value of the power supply voltage falls below a second threshold larger than the first threshold, the processor transmits an instruction related to applying a brake to the movement of the magnetic head to the motor driver.