Surface light source device having specific structure; lighting device and backlight device containing the same

A surface light source device, comprising:

• an organic electroluminescent element including a luminescent layer;
• and a light-emitting surface structure layer disposed in contact with at least one of surfaces of the organic electroluminescent element, wherein:
• the light-emitting surface structure layer includes a concave-convex structure provided on a surface thereof on a side toward a light-emitting surface of the device,
• 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 comprises a first diffusing member and a second diffusing member on which light emitted from the luminescent layer is incident,
• the first diffusing member is provided as a layer constituting all or part of the light-emitting surface structure layer, and is a member that allows the incident light to pass through in a diffused manner, and
• the second diffusing member is disposed at a position farther from the light-emitting surface structure layer than the organic electroluminescent element, and is a member that reflects the incident light in a diffused manner.