NOVELTY - A biaxially oriented polyester film has thickness of 30-500 mu m, degree of plane orientation ( Delta P) of 0.150-0.180 and ratio of surface axis orientation degree between front and back surfaces of the film of 0.80-0.98. The ratio of the surface axis orientation degree is calculated by measuring the in-plane rotation every 10 degrees , in the film sample for 0-170 degrees . The maximum or minimum value in 18 points measured in the front and back surfaces of the film sample is defined as the surface axis orientation degree (Ymax and Ymin).    USE - Biaxially oriented polyester film for use in curable resin laminated material (claimed) for reflecting plate, diffusion sheet, lens sheet, hard coat film and liquid crystal display device. Uses include but are not limited to touch panel, protective film for solar cell, polarizing plate and polarizer, solar cell back sheet, base film for recording material and building material.    ADVANTAGE - The biaxially oriented polyester film has excellent planarity and surface accuracy.    DETAILED DESCRIPTION - A biaxially oriented polyester film contains polyethylene terephthalate type resin and has thickness of 30-500 mu m, degree of plane orientation ( Delta P) of 0.150-0.180 and ratio of surface axis orientation degree between front and back surfaces of the film of 0.80-0.98. The ratio of the surface axis orientation degree is calculated by measuring the in-plane rotation every 10 degrees , in the film sample for 0-170 degrees , after measuring light absorbency (A1340) in the wavelength of 1340 cm-1 and light absorbency (A1410) in the wavelength of 1410 cm-1 by polarized light attenuated total reflectance (ATR) method. The maximum or minimum value in 18 points measured in the front and back surfaces of the film sample is defined as the surface axis orientation degree (Ymax and Ymin). The ratio is measured using the larger value in the denominator. An INDEPENDENT CLAIM is included for curable resin laminated material, which consists of cured resin layer laminated on the biaxially oriented polyester film used as base film.