A vertical semiconductor device with which improvements in pinch-off characteristics and withstand voltage performance can be stably obtained by reliably fixing the potential of a p-type GaN barrier layer is provided. The semiconductor device comprises: a regrowth layer (27) having an opening (28) provided to a GaN laminate (15), and including a channel positioned in such a manner as to cover the wall surface of the opening; an n+ source layer (8) that comes into ohmic contact with a source electrode (S); a p-type GaN barrier layer (6); and a p+ GaN auxiliary layer (7) positioned between the n+ source layer (8) and the p-type GaN barrier layer (6). The semiconductor device is characterized in that the p+ GaN auxiliary layer (7) forms a tunnel junction with the n+ source layer (8) in order to fix the potential of the p-type GaN barrier layer (6) to the source potential.