To provide a surface light source which has high luminance and is excellent in uniformity, in light diffusion from the direct lower type surface light source and in luminance characteristics by finding an optical diffusion plate attaining high luminance while being excellent in light diffusion and using the optical diffusion plate.

The direct lower type surface light source has a construction with at least a plurality of linear fluorescent tubes aligned in parallel and a reflection plate and the optical diffusion plate respectively disposed on the rear side and on the front side of the fluorescent tubes. The optical diffusion plate contains heteromorphic particles of which the maximum diameter and the minimum diameter are different from each other and further is characterized by being an optical diffusion plate of which the principal axis of orientation, being a mean direction of directions of the maximum diameters of the heteromorphic particles, is directed to an in-plane direction of the film.