A semiconductor device for adequately removing heat generated by a semiconductor element is provided. A semiconductor device 100 is equipped with a substrate 2, having a bottom surface 2b and an element mounting surface 2a which is positioned on the opposite side of bottom surface 2b, and a semiconductor element 1, having a main surface 1a which is mounted onto element mounting surface 2a. With L being the length in the long direction of main surface 1a and H being the distance between bottom surface 2b and element mounting surface 2a, the ratio H/L is 0.3 or greater. When the semiconductor element is a light emitting element, element mounting surface 2a is a cavity 2u, and element 1 is provided in cavity 2u. A metal layer 13 is provided on the surface of cavity 2u.