A semiconductor device which can prevent peeling off of a gate electrode is provided. The semiconductor device has GaN buffer layer 12 formed on substrate 11, undoped AlGaN layer 13 formed on this buffer layer 12, drain electrode 16 and source electrode 17 formed separately on undoped AlGaN layer 13, which form ohmic junctions with undoped AlGaN layer 13. Between drain electrode 16 and source electrode 17, insulating layer 20 which has opening 19 is formed, and metal film 21 is formed on a surface of insulating layer 2. Gate electrode 18 which forms a Schottky barrier junction with undoped AlGaN layer 13 is formed in opening 19, and gate electrode 18 adheres to metal film 21.