An organic electroluminescent light source device includes, in the following order from a light-emitting surface side, a first transparent electrode layer, a luminescent layer, a second transparent electrode layer, and a reflecting layer, wherein the reflecting layer includes a concavo-convex structure with an average inclination angle of 12 to 45°. An organic electroluminescent light source device includes, in the following order, a first transparent electrode layer, a luminescent layer, a second transparent electrode layer, a diffusing layer, and a reflecting-scattering layer, wherein the diffusing layer has a concavo-convex surface including a concavo-convex structure with an average inclination angle of 17 to 45°.