A nitride semiconductor transistor device provides a normally-off nitride semiconductor transistor device which is excellent in switching properties with less dispersion of the properties. The nitride semiconductor transistor device has a buffer layer, a GaN layer, and an AlGaN layer in turn grown on a substrate. A first insulating film, a charge storage layer, a second insulating film, and a control electrode are in turn grown on the AlGaN layer. A source electrode and a drain electrode are formed to sandwich the charge storage layer over the AlGaN layer. A threshold voltage to shut off an electric current flowing between the source and drain electrodes through a conductive channel induced at an interface of the AlGaN layer and the GaN layer is made positive by adjusting the charge stored in the charge storage layer.