An organic electroluminescent light source device including a first transparent electrode layer, a light-emitting layer, a second transparent electrode layer, and a reflective layer having a reflective surface, in this order from a light-emitting surface side, and further including a structural layer X that is provided between the second transparent electrode layer and the reflective surface and is in contact with the reflective surface, wherein the reflective surface has a concavo-convex structure, the concavo-convex structure has a plurality of concavo-convex structure units formed of depressions or protrusions, and a refractive index n of the structural layer X, an inclination angle θ×1 (°) of the concavo-convex structure units, and a mean inclination angle θ×2 (°) of the concavo-convex structure at the reflective surface satisfy θ×1≦sin−1(1/n) and {90−sin−1(1/n) }/3≦0×2≦sin−1(1/n).