According to one embodiment, a monitoring unit of a magnetic disk device monitors a supply voltage supplied to a plurality of actuators by a first threshold value at which power supply by back electromotive force of a motor is started, and a second threshold value that is larger than the first threshold value. In a case where the supply voltage is lower than or equal to the second threshold value and is higher than the first threshold value, a controller interrupts execution of processing of reading/writing data to a magnetic disk in at least one of the actuators according to a predetermined condition based on execution status of the processing in the actuators.