In a diffusion sheet 101, a plurality of approximately trapezoidal columnar unit lens portions 11 are disposed such that the long-axis directions thereof are in parallel with each other, and all the surfaces of the unit lens portions, which correspond to the long bottom segments 112 of the approximately trapezoidal sections of the unit lens portions 11 are disposed on a light-incident-side flat surface. Further, light absorbing portions 12 are disposed in the grooves between adjacent unit lens portions 11 to absorb and/or shield external light incident from a light outgoing side. In the diffusion sheet 101 arranged as described above, light incident on the unit lens portions 11 from a light incident side is totally reflected on the surfaces corresponding to the side segments 111 of the approximately trapezoidal sections vertical to the long-axis direction of the unit lens portions and outgoes from the light outgoing side. In each of the unit lens portions 11 arranged as described above, the distance h between both the bottom segments 112 and 113 of the approximately trapezoidal section is 120% or more to 400% or less of the length p of the long bottom segment 112 (h/p=1.20 to 4.00).