There is provided a white polyester film which retains practically satisfactory reflectivity with respect to the visible light range, can undergo stable film production even when inorganic fine particles are added in high concentration and is stable against a dimensional change caused by heat generated from a light source. The polyester film is a polyester film which comprises a composition comprising a copolyester and 30 to 50 wt % of inorganic fine particles based on the composition, shows a thermal shrinkage at 85° C. in longitudinal and transverse directions of not higher than 0.7% and a thermal shrinkage at 150° C. in the longitudinal and transverse directions of not higher than 5.0%, and shows an average reflectance at a wavelength of 400 to 700 nm of not lower than 90%.