For obtaining a flying height of a magnetic head from a magnetic disk, the magnetic head being placed in a slider arranged at an interval with the magnetic disk, an initial setting process and a flying height detecting process are performed. In the initial setting process, driving power to a heater is increased gradually, from a state where the heater arranged at a position in proximity to the magnetic head in the slider is not driven, until the magnetic head makes contact with the magnetic disk. Then, in each stage, an electrical resistance value of the sensor arranged at a position in proximity to the magnetic head in the slider, which is increased due to the heat from heater, and either an approach distance of the magnetic head toward the magnetic disk or the flying height of the magnetic head from the magnetic disk are acquired. Then, basic data is prepared by obtaining the relationship between a variation of the electrical resistance value of the sensor and the flying height of the magnetic head from the magnetic disk, which is obtained in each stage or which calculated from the approach distance in each stage. In the flying height detecting process, an electrical resistance value of the sensor is determined, and a variation of the electrical resistance value is calculated from the determined values. Then, the flying height of the magnetic head from the magnetic disk in the state where the electrical resistance value was determined is obtained based on the basic data obtained in the initial setting process, using the calculated variation of the electrical resistance value.