A biaxially oriented polyester film has been granted a patent. The film has a Young’s modulus in a longitudinal direction of 8 to 12 Gpa; a Young’s modulus in a transverse direction of 6.5 to 9 Gpa; a temperature expansion coefficient in the transverse direction (·t) of -5x10-6/° C to +12x10-6/° C; and a humidity expansion coefficient in the transverse direction (·h) of +6x10-6/% RH to +12x10-6/% RH. The film also has a thermal shrinkage factor at 105° C in the transverse direction of 0 to +1.5% and comprises ethylene-2,6-naphthalene dicarboxylate in an amount of at least 95 mol % of the total of all the recurring units.