To provide a biaxially oriented polyester film which is excellent in peelability, flexibility, and others and especially suitable for a photoresist cover.

A biaxially oriented polyester film having trimethylene terephthalate units of at least 50 mole % of the total repeating units is used as a cover film having Young's modulus of 50-450 kg/mm2 in both machine direction(MD) and transverse direction(TD), at least 5% in haze value, and 12% or below in thickness mottle.