To provide a luminance increasing sheet product used for an alternative product to BEF and DBEF, preventing yellowing deterioration of a liquid crystal caused by effects of light of ultraviolet region wavelength emitted from a light source of a back light, capable of stably maintain a color tone and reducing chromatic variation among luminance increasing sheets to achieve the stable color tone.

An ultraviolet absorber is included in an inner layer of a biaxially-stretched polyester film comprising at least three layers and 75-350 μm of thickness. In the biaxially-stretched polyester film for a luminance increasing sheet member, light transmittance at 370 nm of wavelength is 2.0% or less and haze is 2.3% or less. A heating contraction coefficient in the film longitudinal direction (MD) and a heating contraction coefficient in the film width direction (TD) after treatment for thirty minutes at 150°C are 1.5% or less and 0.7% or less, respectively. A color tone reflection method y value when the plurality of films are laminated so that total measured thickness is 900-1,100 μm and comes closest to 1,000 μm is 0.3230 or less.