To provide an optical sheet which can efficiently guide the beams from a light source to a liquid crystal screen and which makes a back light unit possible to be small-sized.

The optical sheet 1 has a structure of a base part 2 in the lower side and a light diffusing part 4 in the upper side which are tightly fit with each other in the vertical direction without a gap. The light diffusing part 4 contains beads 5b dispersed in a binder 5c. The refractive index of the binder in the light diffusing part 4 is controlled to be lower than the refractive index of the base part 2. The base part has a large number of triangular parts 3 each having a first side face 3a and a second side face 3b. The first side face 3a is formed with an angle larger than the angle made between the peak direction of the distribution of a beam L1 entering the lower face of the base part 2 and refracting and propagating in the base part and the lateral direction, while the second side face 3b is formed almost perpendicular to the lateral direction.