There is provided a very thin light guide plate for a surface light source device and a method of manufacturing it without the use of high-precision molding machine. A surface light source device using this light guide plate generates outgoing light having high uniformity. According to the manufacturing method, a light guide plate free from a weld line or warp can be manufactured with good transferring characteristics. The light guide plate is thick on an incident surface 1 side and thin on a lower surface 4 side. At the central portion of an incident surface 1 in a longitudinal direction, a projecting portion obtained by cutting an overhang portion 7 at a position a distance D apart from the incident surface 1 is formed. The cut surface of the projecting portion is not made specular and is kept rough. In molding of the light guide plate, a molten material is supplied from the position of a gate mark 9. After molding, the overhang portion 7 is cut off by an ordinary cutter, thereby obtaining a light guide plate.