A semiconductor light emitting device comprises an element that emits light and a substrate on a main surface of which the element is mounted. The main surface of the substrate composed of two areas, (i) a mount area which is rectangle and on which the element is mounted, and (ii) a pad area that is equipped with a pad for wire bonding. The pad area is contiguous to the mount area on one side of the mount area, and the pad area decreases in width continuously or stepwise in a direction away from the one side.