Disclosed is a biaxially oriented polyester film, which is a polyester film having a polyester layer (P1 layer) containing a polyester (A1) comprising either ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main constituent, a thermoplastic resin (D1), and inorganic particles (C1), wherein the content of the thermoplastic resin (D1) in the P1 layer, W D1 , is not less than 2% by mass and not more than 40% by mass based on the P1 layer; the relationship: 1.5 × Mw A1 ' / Mw A1 ‰¤ Mw D1 ' / Mw D1 is satisfied, wherein Mw A1 is the weight-average molecular weight of the polyester (A1); Mw D1 is the weight-average molecular weight of the thermoplastic resin (D1); Mw A1 ' is the weight-average molecular weight of the polyester (A1) after treatment at 125°C and 100% RH for 72 hr; and Mw D1 ' is the weight-average molecular weight of the thermoplastic resin (D1) after treatment at 125°C and 100% RH for 72 hr; and, in the P1 layer, the thermoplastic resin (D1) is present in the polyester (A1) as dispersion phases, and the number of the dispersion phases having a longitudinal length of more than 30,000 nm (30 µm) is not more than 2/3 × 10 9 nm 2 (2/3,000 µm 2 ). The biaxially oriented polyester film of the present invention has excellent moist-heat resistance and UV light resistance, and a solar battery with high durability can be provided by using this in a solar battery back sheet.