The invention provides a flame-retardant biaxially-oriented polyester film which is porous, and has high reflectance. The flame-retardant biaxially-oriented polyester film contains a polymer component containing polyethylene terephthalate and a flame retardant. The polyester film has an intrinsic viscosity of 0.50 to 0.64 dL/g and a density of 1.21 to 1.27 g/cm3. A content of the polyethylene terephthalate in the polyester film is 70 to 97% by mass. The flame retardant contains at least one phosphorus-based flame retardant selected from the group consisting of a phosphinate and a diphosphinate. A content of the phosphorus-based flame retardant in the polyester film is 3 to 8% by mass. The polyester film is a porous film having an average reflectance of 60 to 74% at a wavelength of 400 to 700 nm. The polyester film has a thickness of 15 to 45 μm.