The present invention relates to a magnetic disk device and provides a magnetic disk device in which data recording areas are separately set based on the inner recording boundary or outer recording boundary of the head measured for each magnetic disk device. When the recording linear (angular) density that has to be attained to ensure a prescribed capacity cannot be attained by the head, the recording linear (angular) density that has to be attained by the head is reduced and the prescribed capacity is realized by expanding the data recording area. A method using a counter-electromotive force generated when the head is moved toward a stopper or ramp load or a method by which the measurements are conducted based on the head position information from the servo signal can be used for measuring the positions of the inner recording boundary and outer recording boundary of the head.