A polyester film suitable for use in battery packages, particularly for use in deep drawn packages of lithium ion secondary batteries that includes an electrolyte (in a solid state or in a liquid state), and which is also suitable for use in packages of pharmaceutical products that have various shapes. The polyester film is characterized in that the stresses at 5% elongation (F5 values) and the stresses at 10% elongation (F10 values) of the film in the longitudinal direction and in the transverse direction at 25° C., respectively, satisfy (I) and (II): 1.5≧F10MD/F5MD≧1  (I) 1.5≧F10TD/F5TD≧1  (II)