The trim structure suppresses an increase in the effective track width due to expansion of a magnetic flux generated during writing in a narrow track. [0079] Next, as shown in FIG. 9A and FIG. 9B, a protection layer 16 made of an insulating material such as Al2O3 or SiO2 and having a thickness of 5 to 50 ??m, for example, is formed over the entire surface through sputtering, for example.