Causing the growth of a GaN material with respect to a sapphire substrate using a conventional technique is inevitably followed by the occurrence of dislocations. Using a mask layer results in that the dislocations laterally flow. However, since the GaN crystal collides with a semiconductor layer that laterally grew from an adjacent region, perfect elimination of the dislocations is impossible. In view thereof, the invention is intended to provide a nitride compound-based semiconductor light emitting device which is based on using semiconductor layers that have been formed in a state of the dislocations' being less existent therein and which therefore has excellent property. To solve the above-described problems, the invention provides a semiconductor light emitting device being a gallium nitride-based semiconductor light emitting device that not only is equipped with a substrate but is also equipped with at least a first conductivity type semiconductor layer, active layer, and second conductivity type semiconductor layer in this sequential order on the substrate, wherein the first conductivity type semiconductor layer has a level difference portion the levels of which have a spacing therebetween in the lamination direction; and the dislocation density of the active layer that is formed on a portion of the first conductivity type semiconductor layer that has the higher level is lower than that of the active layer that is formed on a portion thereof that has the lower level.