A plate-shaped light guide for a planar light source device that guides a light emitted from a primary light source includes a light incident end face, a light emitting face and a back surface on the opposite to the light emitting face, wherein a plurality of first elongated lenses having an average slant angle of 0.5 to 60.degree. are continuously or intermittently formed on one of the light emitting face and the back surface, and at least a portion of a surface of each first elongated lens, or at least a portion of the other one of the light emitting face and the back surface, or at least the portion of the surface of each first elongated lens and at least the portion of the other one of the light emitting face and the back surface includes a rough surface having an average slant angle of 0.1 to 8.degree..