PURPOSE: To obtain a polyester film having excellent mechanical strength by improving the casting step before the orienting step of a biaxially oriented polyester film in which double refraction and the other characteristics are specified. CONSTITUTION: A cast film formed by melt extrusion by using a flat die has a double refraction Δn of 0.2×10 Δn , and a cast polyester film simultaneously satisfies a formula (a+b)/2C>=1.25 and a formula a/b>=1.20, where a, b and c are CP/HAS spectrum of benzene ring carbon at positions l, 3 in a structural analysis by a solid state high resolution NMR, and indicate the heights in which a background is subtracted from the peak height at the position of A=134ppm and C=131.6ppm of the CP/MAS spectrum.