The track width of a magnetic head is specified by the interval between a pair of grooves in the slider surface that are formed by a projection ion beam. Specifically, the grooves are processed by projecting a mask having the pattern of the pair of grooves with a projection ion beam with reduction to a magnification of 1/10, for example. Although the peripheral strain of the pattern thus formed is large and the current distortion is not uniform when the projection ion beam batch irradiation area is limited, the increase in depth of the grooves formed at the periphery of the grooves does not affect the track narrowing process at the center portion of the projection ion beam since the strain thereof is negligibly small. As a result, a narrow track width is processed with high precision.