A surface light source device of side light type illuminating an LCD panel and comprising a guide plate, a primary light source, a reflection sheet, and a prism sheet for light control. A plurality of fine light scattering elements and rough area M are formed on an emission face of the guide plate. The rough area M is formed among the light scattering elements and has scattering power weaker than that of the light scattering elements. The dot-like light scattering elements are distributed according to a certain pattern that is designed to promote emission. Emission is promoted where the light scattering elements are formed and also in the area M. Accordingly, fine periodic bright-dark unevenness and glaring are reduced and the prism sheet, disposed on the emission face, is prevented from sticking to the emission face.