The present invention provides a light guide plate and a surface light source using the light guide plate to improve the utilization efficiency of external light and light from an internal light source and to achieve thinner width of the light guide plate. In this invention, internal light emitted from an internal light source 3 is brought to the inside of the light guide plate 1 from an internal light collecting surface 16 in the light guide plate 1, and external light such as natural light is brought to the inside of the light guide plate 1 from an external light collecting portion 11. The light brought to the inside of the light guide plate reflects and is converged several times from a reflective plate 15 mounted to the outside of the light reflective surface 14 and three side end surfaces 12 and is emitted from the light emissive surface 13 of the light guide plate 1 to a diffusing plate 6. The light incident to the diffusing plate 6 is diffused uniformly within the desired angle range by the diffusing plate 6 and a lens sheet 7, and is emitted from the lens sheet 7 as synthesized light.