A compound semiconductor element is provided which electrically connects an electrode 3 formed on one main surface 2a of a compound semiconductor region 2 with a substrate 5 to fix an electric potential of substrate 5 at an electric potential of electrode 3, thereby preventing fluctuation in electric potential of substrate 5 under the changing operating condition of the device for stabilization in electric property of the device. Also, formed between compound semiconductor region 2 and substrate 5 is an insulating layer 6 for blocking a leakage current which may flow longitudinally between one main surface 2a of compound semiconductor region 2 and substrate 5 so that sufficiently high withstand voltage property can be given between compound semiconductor region 2 and substrate 5. In addition, formed in compound semiconductor region 2 is a notch 14 which extends in the thickness direction from main surface 2a of compound semiconductor region 2 and reaches at least insulating layer 6, and an insulating protective layer 15 covers a side surface of a conductive film 7 exposed to the notch 14 to prevent occurrence of electric discharge between conductive film 7 and substrate 5 for stable and high withstand voltage.