A nitride semiconductor device has: a substrate; a semiconductor lamination formed on the substrate, and including a channel layer of nitride semiconductor; source and drain electrodes formed on the semiconductor lamination in ohmic contact with the channel layer; an insulating layer formed on the semiconductor lamination, and having an opening in a gate electrode contact area, a total thickness portion having a flat surface and a total thickness in an area spaced apart from the opening, and a transient portion with monotonically changing thickness between the opening and the total thickness portion, a sidewall of the insulating layer facing the opening rising steeply to a partial thickness of the total thickness; and a T-shaped gate electrode contacting the semiconductor lamination layer in the opening and extending on the insulating film to portions with increased thickness thicker than the partial thickness.