A semiconductor device includes: a first GaN based semiconductor layer; a second GaN based semiconductor layer disposed on the first layer and having a bandgap larger than that of the first layer; a first electrode disposed on the second layer; a second electrode disposed on the second layer; a p-type third GaN based semiconductor layer disposed between the first electrode and the second electrode on the second layer; a third electrode disposed on the third layer; a p-type fourth GaN based semiconductor layer disposed directly on the second layer and disposed separated from the third layer; a first insulating film disposed on the fourth layer; and a first field plate electrode disposed interposing the first insulating film in a space with the fourth layer, the first field plate electrode being separated from the fourth layer, and the first field plate electrode electrically connected to the first electrode.