A surface light source device has a light emitting surface and includes a light guide plate having a light exit surface and a pair of light entrance surfaces. When the angular distribution of luminance in a plane parallel to a first direction is measured on the light emitting surface at varying positions along the first direction, the peak angle θc in the angular distribution of luminance at a center position Pc in the first direction lies between the peak angle θa in the angular distribution of luminance at an end position on one side in the first direction and the peak angle θb in the angular distribution of luminance at an end position on the other side in the first direction.