In a surface light source device, a light guide plate includes a light introduction part that traps the light emitted from a point light source and incident from a light incident surface, and a light guide plate body. The light introduction part includes an inclined surface inclined from a surface in a portion having a thickness larger than that of the light guide plate body toward an end of the surface of the light guide plate body. The light guide plate body includes a directivity conversion pattern located between an effective lighting region of the light guide plate body and the light introduction part. The directivity conversion pattern converts a directivity direction of the light from the light introduction part to the effective lighting region such that an angle formed by the directivity direction and a direction perpendicular to the light incident surface decreases when viewed from above.