To obtain a polyester film that has excellent barrier properties, flexibility and is suitable for packaging by constituting the film of a specific biaxially oriented polyester and specifying the refractive index in the thickness direction and the density.

The objective polyester film is constituted of a biaxially oriented polyester film comprising ≥80 mol.% of the trimethylene terephthalate recurring units, with a refractive index in the thickness direction nz of ≤1.570, preferably 1.420-1.570, a density of ≥1.315 g/cm3, preferably 1.325-1.365 g/cm3, preferably a surface roughness Ra of 2-100 nm, a haze value of ≤12%, a Young's modulus of ≥110 kg/mm2 in both the machine and transverse directions. The poly(trimethylene terephthalate) is prepared by using terephthalic acid as a main acid component and 1,3-propane diol as a main glycol component.