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 the thickness of a translucent substrate substantially in parallel in the vicinity of a primary light source and gradually reducing the thickness of the substrate in accordance with separation from the light source.

A roughened surface part 7 is formed on at least one of the front face 4 of a transparent substrate 2 and its reflection face, i.e., a rear face, in order to project light from the front face 4. For instance, the roughened surface part 7 is formed as an elliptic shape, the size of the ellipse is increased and the area of a smoothed part 8 is reduced in accordance with separation from the light source 1 to set up the quantity of scattered reflection to a high value. The substrate 2 is constituted so as to arrange its front face and rear face substantially in parallel in the vicinity of a primary light source 1 and gradually reduce the thickness of the substrate 2 in accordance with separation from the light source 1. The substantially parallel part between the front and rear faces is preferably set up to a range from the same length as the thickness of the substrate 2 up to five times the thickness of the substrate 2.