A magnetic head slider includes a near-field light generator, an incorporated heater is activated to thermally-expand the magnetic head slider so that a first protrusion is generated on the air bearing surface, and a second protrusion protruding from the first protrusion is generated by a thermal expansion of the near-field light generator. A standard signal is written to a recording medium with a predetermined magnetization. A relation between a residual magnetization of the standard signal and a power of the heater is obtained by lowering the magnetization of the standard signal by heating the recording medium with the near-field light while light output of laser light is maintained to be constant and the power of the heater is varied. Further, a relation between the first spacing and the residual magnetization is obtained. A value of the first spacing is obtained as a protrusion height of the second protrusion from the first protrusion when the second protrusion contacts the recording medium, the value of the first spacing being determined where an absolute value of a change rate of the residual magnetization is less than a predetermined standard value.