A nitride semiconductor device is disclosed, where the nitride semiconductor device is a type of field effect transistor having a gate electrode and an insulating film covering the gate electrode. The gate electrode has stacked metals of nickel (Ni) and gold (Au), while, the insulating film is made of silicon nitride (Si). A feature of the gate electrode of the present invention is that the nickel layer contains silicon (Si) atoms at an atomic concentration from 0.01 at % to 10 at %.