The semiconductor device of this invention includes an active region formed from a group III nitride semiconductor grown on a substrate and an insulating oxide film formed in a peripheral portion of the active region by oxidizing the group III nitride semiconductor. At least about 5% of the total heat output produced by combustion of the first-stage fuel and the first-stage oxidant is removed from the primary combustion zone, forming cooled first-stage combustion products. This prevents the recording magnetic field strength from decreasing at the track edge and increases the effective track width while suppressing side writing, thereby realizing a magnetic recording disk apparatus with a high track density. The second rack is fixed on the second plate for actuating the gear assembly.