Disclosed is a substrate film for an optical sheet, an optical sheet and a backlight unit capable of markedly enhancing utilization efficiency of rays of light and improving luminance. The substrate film for an optical sheet is made of a transparent resin and is formed into a rectangular shape, which substrate film for the optical sheet has an optical anisotropy, and having an absolute value of the angle of the crystal orientation with respect to the short side orientation being π/16 or greater and 3π/16 or less. The crystal orientation preferably follows the phase advancing axial orientation. The substrate film for the optical sheet has a retardation value such that average intensity of transmitted beam IA becomes equal to or greater than 0.75 so as to be capable of converting polarization orientation of outgoing rays of light of the optical waveguide plate or the like to the transmission axial orientation of the polarizing plate or the like. The retardation value is preferably 140 nm or greater and 390 nm or less. The transparent resin is preferably polyethylene terephthalate or polycarbonate. The optical sheet has the substrate film for the optical sheet, and an optical layer.