Abstract: A surface light source device of side light type has a scattering guide plate, a primary light source, a reflection sheet and a prism sheet. An emission surface of the guide plate provides a light control surface. The light control surface has many fine projections approximately perpendicular to an incidence surface or inclined by a small angle with respect to the incidence surface. In each of the projections, dullness varying in accordance with position on the emission surface is given to a slope-couple including a pair of slopes produce a uniform emitting intensity from the emission surface. A back surface of the guide plate may provide a light control surface so that dullness varying in accordance with position is given to slope-couples.