There is provided a semiconductor device comprising a group III nitride semiconductor layer; a gate insulating film formed on the group III nitride semiconductor layer; and a gate electrode formed on the gate insulating film. The gate insulating film comprises a first film that is placed on the group III nitride semiconductor layer, includes silicon and has a higher crystallization temperature than a crystallization temperature of aluminum oxide; and a second film that is placed on the first film and contains aluminum oxide. The first film has a hydrogen concentration of not lower than 1×1021 atoms/cm3, a nitrogen concentration of not lower than 1×1019 atoms/cm3 and a carbon concentration of not lower than 1×1019 atoms/cm3. This configuration prevents crystallization of aluminum oxide.