Biaxially oriented polyester film, process for producing the same, and use thereof as substrate for photographic sensitive material

A method of producing a biaxially oriented polyester film whose thermal shrinkage factors satisfy relationships represented by the following expressions (1) to (5) at the same time when it is heated at 120° C. for 20 seconds:

• 0.001≦SMD≦0.1 (1)
• −0.1≦STD≦0.1 (2)
• −0.05≦S45≦0.05 (3)
• −0.05≦S135≦0.05 (4)
• |S45−S135≦0.05 (5)
• wherein SMD is a thermal shrinkage factor (%) in machine direction of the film under the above conditions, STD is a thermal shrinkage factor (%) in a direction perpendicular to that direction under the above conditions, and S45 and S135 are thermal shrinkage factors (%) in a 45° direction and a direction perpendicular to the 45° direction under the above conditions when the machine direction of the film is 0° and a direction perpendicular to that direction is 90°, respectively, the method comprising thermally relaxing at least once a biaxially oriented polyester film whose thermal shrinkage factors do not satisfy at least one of the above expressions (1) to (5) when it is heated at 120° C. for 20 seconds while it is suspended downward by its own weight.