A surface light source device has a point light source disposed opposite a light incident surface of a light guide plate. The light guide plate includes a light introduction part, and a light guide plate body having a thickness smaller than a maximum thickness of the light introduction part. The light introduction part includes an inclined surface inclined from a surface in a portion having a thickness larger than the thickness of the light guide plate body toward an end of the surface of the light guide plate body. The light guide plate body has a light exit pattern for reflecting the light in the light guide plate body to output the light from a light exit surface. In a region close to the point light source of the light guide plate body, the thickness of the light guide plate body increases gradually with distance from the point light source.