In a manufacturing process of a head slider according to one embodiment, a flare point height of a main pole is estimated according to the following procedure. Magnetic field intensity is measured by a sensor, and a maximum point of the magnetic field is assumed as a center of the main pole. The measurement is moved from the center point along a Z direction, such that a measurement value corresponds to a reference value. An attenuation rate (a gradient) of a measurement value of magnetic field intensity to displacement in the Z direction of the measurement point is calculated and normalized with the measurement value, and plotted with respect to the measurement value itself. The gradient is compared to a gradient in a database of samples being separately prepared, each sample having known flare point height, allowing an estimation of the flare point height of the said sample from the comparison.