A nitride semiconductor device of the present invention includes: a nitride semiconductor laminated structure comprising an n type first layer, a second layer containing a p type dopant laminated on the first layer, and an n type third layer laminated on the second layer, each layer of the nitride semiconductor laminated structure made of a group III nitride semiconductor, and the nitride semiconductor laminated structure formed with a first trench and a second trench, the first trench penetrating the second layer from the third layer and reaching at least the first layer, and the second trench having a side wall extending from the first, second, to third layers and being different from the first trench; a surface insulating film containing at least silicon nitride formed such that the surface insulating film covers the surface of the first trench; a gate insulating film formed on the side wall of the second trench such that the gate insulating film extends over the first, second, and third layers; and a gate electrode formed such that the gate electrode is opposed to the side wall of the second trench with the gate insulating film sandwiched between the gate electrode and the side wall.