An optical sheet comprising: a transparent substrate having a first surface S1 and a second surface S2; a first light diffusion layer provided on a first surface S1 side of the transparent substrate; and a second light diffusion layer provided on a second surface S2 side of the transparent substrate, wherein a haze x1 (%) of the first light diffusion layer and a haze x2 (%) of the second light diffusion layer satisfy f(x1)/(f(x1)+f(x2))≦2/3 (where when x≦88, a function f(x)=1.9×(ln(1−x/90))2, and when x>88, the function f(x)=22.5x−1952.5) and (f(x1)+f(x2))≧12; and a surface light source device having the same.