A semiconductor light emitting device is provided which comprises a metallic support plate 1; a semiconductor light emitting diode chip 2 mounted on a support surface 13a defined on an upper surface 13 of support plate 1; wiring conductors 4 mounted on side and upper surfaces 12, 13 of support plate 1 via insulators 6; and a plastic encapsulant 3 for sealing side and upper surfaces 12 and 13 of support plate 1 and a part of wiring conductors 4. As support plate 1 comprises at least one projection 16 extending from side surface 12 of support plate 1 through a notch 3a formed in plastic encapsulant 3 for outward exposure, heat from diode chip 2 can be efficiently diffused to the outside through projections 16 of support plate 1 extending through notch 3a of plastic encapsulant 3 for outward exposure, when heavy current flows through diode chip 2 through wiring conductors 4 for stronger lighting. Also, the device can selectively be mounted on a circuit board in either of the horizontal and vertical mounting structures by selectively disposing bottom surface 14 or projections 16 of support plate 1 in face of a mounting surface of circuit board and connecting lead terminal 4d of wiring conductor 4 to circuit board to direct light from diode chip 2 perpendicularly or parallel to circuit board.