To form a surface light source device capable of effectively utilizing light from a light source and suppressing the generation of a bright line on an incident part by forming a roughened surface part on the end face of a translucent substrate faced to a primary light source.

At least one end face of a translucent substrate (transparent substrate) 2 is used as an incident light face 5 and a face rectangular to the face 5 is used as an outgoing light face (front face) 4. A primary light source 1 such as a fluorescent lamp is arranged approximately in parallel with the inclident light face 5, reflection faces 3 consisting of a metallic layer such as silver or aluminium are formed on the end face other than the face 5 and a rear face and a roughened face part 7 is formed on at least one of the outgoing light face 4 and its opposite side face (rear face). Incident light made incident from the face 5 is advanced through the substrate 2, scattered on the roughened surface part 7 and projected from the face 4 as outgoing light. A reflection cylinder 6 is arranged on the periphery of the light source 1 except the incident light face 5 to reflect light emitted from the light source 1 to the side of the face 5.