PURPOSE: To provide the titled film which has a high polymn. degree and low extractability of low-molecular compd. from the film surface and exhibits less occurrence of drop out in a magnetic tape, by incorporating a bis-oxazoline into a linear satd. polyester.

CONSTITUTION: A mixt. consisting of a substantially linear satd. polyester (A) (e.g. a satd. polyester derived from terephthalic acid and tetramethylene glycol) and 0.1W10wt% (based on the weight of the component A) bis-oxazoline of the formula (wherein R1WR8 are each H, a hydrocarbon group) such as 2,2'-bis(2-oxazoline or 2,2'-bis(4,4'-dimethyl-2-oxazoline), is melt-extruded under heating at a temp. of from the m.p. of the component A to 320°C into film which is then stretched to produce an oriented polyester film having an intrinsic viscosity of 0.55 or above. The extractability of low-molecular compd. from this film with xylene solvent is not more than 1wt%. Absorbance at a wavelength of 248nm is 0.30 or below, when the soln. of low-molecular compd. extracted from the film surface with chloroform solvent is measured with a spectrophotometer.