Abstract: A flying height control device controls a flying height of a magnetic head, while preventing unnecessary execution of flying height control when read performance drops. A control circuit, which controls the flying height by controlling the heat power of a heater element of a magnetic head, checks the read performance, detects a drop in read performance, then judges and discerns which the cause of the drop in read performance is in the magnetic head and the magnetic disk, and executes the flying height control by the heat power correction processing when judging that the cause of the drop in read performance is in the magnetic head. Thus unnecessary adjustment while the magnetic disk device is operating is prevented.