A biaxially oriented polyester film, comprising a polyester containing inert particles, wherein (A) the smallest value of Young's modulus in the planar direction is 5 GPa or more, and the difference between the largest value and the smallest value is 1 GPa or less; (B) the largest value of thermal shrinkage factor when the film is heated at 105° C. for 30 minutes is 0.6% or less, and the difference between the largest value and the smallest value is 0.3% or less; (C) the center line average surface roughnesses (Ra) of the both surfaces of the film are in the range of 2 to 10 nm; and (D) the agglomeration ratio of the inert particles in the film is 30% or less. This film has a flat surface, few large projections formed by the agglomeration of particles, high strength, a small thermal shrinkage factor, rarely experienced an error (D/O), and had excellent head contact and dimensional stability as a base film for flexible disks, particularly high-density magnetic disks.