A polyester film has excellent resistance to hydrolysis, excellent heat resistance in high temperatures and low humidity, and mechanical strength. The polyester film satisfies a stress heat resistant coefficient f(125)≧3 and a wet thermo retention (=100×S(120)/S(0)) of 30% or more. f(125) is a value obtained by substituting t=125° C. in an approximation represented by f(t); t represents a temperature (° C.) at thermo processing; f(t) represents a stress heat resistant coefficient f at the thermo temperature t and represents an approximation to a straight line obtained by linear approximation by a least squares method of values plotted from a relationship between the thermo temperature t and a logarithm (log T(t)) of time T at which a rupture stress is 50% when t is 150° C., 160° C., 170° C., or 180° C.; T(t) is a time (hr) at which the maximum stress in a tensile test after thermo processing at t° C. and 0% RH is 50% of the maximum stress in a tensile test before thermo processing; S(120) is breaking elongation (%) after aging for 100 hours at 120° C. and 100% RH, and S(0) represents a breaking elongation (%) before aging.