The present invention provides a Group III nitride semiconductor light-emitting device exhibiting high-intensity light output in a specific direction and improved light extraction performance. The Group III nitride semiconductor light-emitting device comprises a sapphire substrate, and a layered structure having a light-emitting layer provided on the sapphire substrate and formed of a Group III nitride semiconductor. On the surface on the layered structure side of the sapphire substrate, a two-dimensional periodic structure of mesas is formed with a period which generates a light intensity interference pattern for the light emitted from the light-emitting layer. The light reflected by or transmitted through the two-dimensional periodic structure has an interference pattern. Therefore, the light focused on a region where the light intensity is high in the interference pattern can be effectively output to the outside, resulting in the improvement of light extraction performance as well as the achievement of desired directional characteristics.