The invention provides a polyester film for the protection of a back surface of a solar cell which, when applied to a silicon thin film solar cell, exhibits excellent durability even under high-temperature and high-humidity conditions and long term thermal stability. The polyester film (a) contains a polyester, as a main constituent, obtained by polymerization using a polycondensation catalyst containing aluminum and/or its compound as well as a phosphorus compound having an aromatic group in the molecule; (b) has a whiteness degree of 50 or higher; (c) contains 3 to 50% by mass of fine particles with a mean particle diameter of 0.1 to 3 μm; and (d) has an acid value from not lower than 1 (eq/ton) and not higher than 30 (eq/ton).