Characteristics of a semiconductor device using a nitride semiconductor are improved. A semiconductor device of the present invention includes a buffer layer, a channel layer, a barrier layer, a mesa-type 2DEG dissolving layer, a source electrode, a drain electrode, a gate insulating film formed on the mesa-type 2DEG dissolving layer, and an overlying gate electrode. The gate insulating film of the semiconductor device includes a sputtered film formed on the mesa-type 2DEG dissolving layer and a CVD film formed on the sputtered film. The sputtered film is formed in a non-oxidizing atmosphere by a sputtering process using a target including an insulator. This makes it possible to reduce positive charge amount at a MOS interface and in gate insulating film and increase a threshold voltage, and thus improve normally-off characteristics.