In a manufacturing process of a head slider, a plurality of head elements are formed on a wafer, each head element comprising: a return pole, a coil, and a main pole. The wafer is cut into respective head elements so that individual head sliders are formed. A ratio of an amplitude of an electrical signal applied to the coil of the write head on the head slider to an amplitude of output from an independent magnetic field sensor not embedded in the head slider is calculated, where the independent magnetic field sensor is disposed near the main pole so as to be opposed to the main pole across the air bearing surface, and where the ratio is calculated while a displacement between the main pole and the magnetic field sensor is swept. A flare point height of the main pole is determined from the calculated amplitude ratio.