A surface light source apparatus includes a primary light source (1), an optical guide (3) for guiding light emitted from the primary light source and having a light incoming surface (31) and a light outgoing surface (32), a light deflection element (4) arranged adjacent to the light outgoing surface of the optical guide and having a light incoming surface (41) and a light outgoing surface (42), and a light diffusion element (6) arranged adjacent to the light outgoing surface of the light deflection element. The light incoming surface (41) of the light deflection element has a plurality of prism strings arranged substantially parallel to each other and each having two prism surfaces. At least one of the prism surfaces of these prism strings has a prism surface consisting of at least two planes having different inclination angles. As the plane approaches the light going surface, the inclination becomes larger.