PURPOSE: To decrease the anisotropy of an intra-surface dimensional change by subjecting a biaxially stretched and heat-fixed polyester film to a heat treatment at a prescribed temp. while elongating the film in a transverse direction by a stenter and subjecting the film to a relaxing heat treatment in a contactless state by a floating heat treatment system.

CONSTITUTION: The biaxially stretched and heat-fixed polyester film is first heat treated at the temp. lower than the temp. of the heat fixing while the film is subjected to the elongation, more particularly 1W7% elongation in the transverse direction in the stenter to form the heat-treated film of which the longitudinal shrinkage when the film is held for 72hr under the temp. and humidity conditions of 60°C and 80% RH is 0.05W0.2% and the transverse shrinkage is larger by 0.02W0.2% than the longitudinal shrinkage. While the traveling tension of this heat-treated film is kept under 4.0W10kg/cm2, the film is subjected to the relaxing heat treatment for ≥5sec at 90W110°C in the contactless state to adjust the shrinkage to ≤0.03% in both directions. The dimensional change is thereby decreased and the intra-surface anisotropy thereof is decreased.