A surface light source device of side light type has a guide plate, a primary light source for supplying illuminating light through an incidence surface, first and second edges at which major surfaces respectively intersect the incidence surface, and one or plural additive members arranged in direct contact with the emission surface and/or the back surface thereon in the vicinity of the first edge and/or the second edge. At least one of the additive members has a shield for preventing illuminating light from being incident to the first or second edge. For example, the shield includes absorptive areas formed on the additive members. Each of the absorptive areas is formed so that each of the absorptive areas crosses the first or second edge. A distal end of a reflector arranged in the primary light source or frame blocks incidence of light to the first and/or second edge. The light incidence block to each edge prevents luminance irregularities in the form of a bright line.