In a magnetic head of a high-frequency magnetic field assisted recording system, a width of a high-frequency magnetic field from an oscillator is decreased to enhance an oscillation frequency, in order to realize a high-density recording. An oscillator provided near a main pole, which generates a recording magnetic field, for generating a high-frequency magnetic field is patterned by a conventional photolithography, and then, an oxidation, nitridation, or oxynitridation is performed on the side face in a track width direction. With this process, an oxide layer, a nitride layer, or an oxynitride layer, which is made of a material of the oscillator, is formed on the side face of the oscillator in the track width direction, and the shape of the oscillator is formed to be semi-circular.