A surface light source device includes an organic electroluminescent element including a luminescent layer and a light-emitting surface structure layer disposed on one of the surfaces of the organic electroluminescent element. In the surface light source device, the light-emitting surface structure layer includes a concave-convex structure provided on a surface thereof on the side toward a device light-emitting surface, and the concave-convex structure includes a plurality of concave portions having oblique surfaces and flat portions disposed around the concave portions. The flat portions and/or the concave portions have a size difference in one or more of their width, height, depth, and spacing, the size difference being larger than the difference that causes interference of one or both of emitted light and reflected light.