A surface light source device is provided that has high light extraction efficiency and high mechanical strength and can suppress a change in color tone at different viewing angles. To that end, the surface light source device includes: an organic EL element including a luminescent layer; and a light-emitting surface structure layer that is disposed in contact with one of the surfaces of the organic EL element and defines a concave-convex structure on the surface on the device light-emitting surface side. The concave-convex structure includes a plurality of concave portions having oblique surfaces and flat portions disposed around the concave portions. The surface light source device further includes a diffusing member on which the light emitted from the luminescent layer is incident, the diffusing member allowing the incident light to pass therethrough or reflecting the incident light in a diffused manner.