PURPOSE: To prevent face sags of a ferrite material by applying surface processing on the predetermined face of the monocrystal ferrite material in advance so as to generate a processing affected layer of 2000W3000 and fitting this ferrite material to a polishing jig via a cushion material.

CONSTITUTION: A wafer 1 applied with surface processing so as to generate a processing affected layer of 2000W3000 on the predetermined face of a monocrystal ferrite material in advance is fitted to a polishing jig 4 via a cushion material 8. Abrasive grains are fed while a lapping machine is rotated, the polishing jig 4 is pushed to the lapping machine d5, and the wafer 1 is polished and the processing affected layer is removed. In this case, the whole face of the wafer 1 is brought into contact with the lapping machine 5, thus generation of face sags or partial sags of the wafer 1 is prevented, and the characteristics of a magnetic head can be improved.