A magnetic disk device according to an embodiment includes: a head slider; a magnetic head provided in the head slider; a heater provided in the head slider; and a controller that determines, in seek operation of the magnetic head, a preheat start scheduled position from a remaining seek distance at which a time required for preheating of the head slider can be secured, the controller starting the preheating from the preheat start scheduled position in a case where no bump exists in an area from the preheat start scheduled position to a seek target position, the controller changing a preheat start position to an arbitrary position in an area from the bump to the seek target position in a case where the bump exists in the area from the preheat start scheduled position to the seek target position.