A semiconductor device formed by decreasing thickness of a substrate by grinding, and performing ion implantation. In a diode in which a P anode layer and an anode electrode are formed at a side of a right face of an N− drift layer, and an N+ cathode layer and a cathode electrode are formed at a side of a back face of the N− drift layer, an N cathode buffer layer is formed thick compared with the N+-type cathode layer between the N−-type drift layer and the N+ cathode layer, the buffer layer being high in concentration compared with the N− drift layer, and low compared with the N+ cathode layer. When a reverse bias voltage is applied, a depletion layer is stopped in the middle of the N cathode buffer layer, and thus prevented from reaching the N+ cathode layer, so that the leakage current is suppressed.