To provide a method for producing a biaxially oriented polyester film which has low thermal shrinkage rates in all directions, has high transparency, simultaneously satisfies workability for winding up and carrying, and enables the production of always color shift-free photographic plates, when used for photographic photosensitive materials adopting a thermal developing method.
This method for producing a biaxially oriented polyester film whose thermal shrinkage rates simultaneously satisfy the following expressions: 0.001≤SMD≤0.1...(1), -0.1≤STD≤0.1...(2), -1.05≤S45≤0.05...(3), -1.05≤S135≤0.05...(4), S45-S135≤0.05...(5), [SMD is the thermal shrinkage rate (%) of the film in the longitudinal direction; STD is the thermal shrinkage rate (%) of the film in the lateral direction; S45 and S135 are the thermal shrinkage rates (%) of the film in the directions of 45° and 135, respectively, when the lateral direction and longitudinal direction of the film are 0° and 90°, respectively], when the biaxially oriented film is thermally treated at 120°C for 20 seconds, by subjecting the film to a thermal relaxation treatment in a suspended state once or more.
FUKUDA MASAYUKI
TAGUCHI SUSUMU
