PURPOSE: To improve the recording density and the running property of biaxially oriented polyester film for magnetic recording by performing corona discharge to unoriented film or orienting film made from polyester nearly short of solid grains.

CONSTITUTION: Polyester is a polymer composed mainly of aromatic dicarbonyl component and glycol component such as polyamide, polyolefin, polycarbonate, etc. A stabilizer, color agent and an oxidation inhibitor may be mixed with this polyester but an extremely small amount of solid grains, if contained, can be contained. The average height of projections on the surface of a biaxially oriented film must not exceed 0.015μm. Then, corona discharge treatment is performed to the unoriented film obtained by melting and extruding polyester in the form of a sheet, bringing it into contact with a rotating drum, quenching and hardening it or on the way of uniaxial or biaxial orientation.