An area light source device that is capable of more evenly emitting light and capable of enhancing the light emission luminance by exiting light of narrower directional characteristics. A light guide substrate includes a light introducing section and a light guide plate main body for exiting the light from a light exit surface. A number of diffusion patterns are formed at the light exit surface. A low refraction index layer is formed at the lower surface of the light guide plate main body, and a reflection layer is formed at the lower surface of the low refraction index layer. The point light source is arranged at a position facing the end face of the light introducing section. A tapered portion inclined to gradually increase the thickness of the light introducing section the farther away from the light source is arranged on the upper surface of the light introducing section.