A lower magnetic film, a magnetic gap layer, a conductor coil, an insulating layer for electrically insulating the conductor coil, an upper magnetic film and a protective film are laminated on a substrate. Formation data of a lamination step are collected. An optimum value of a magnetic gap depth G of the magnetic gap layer is calculated on the basis of the formation data. The magnetic gap depth is machined to the optimum value on the basis of the optimum value so calculated.