A polyester resin having excellent dimensional stability against environmental changes and excellent dimensional stability in the processing step while having excellent moldability and a biaxially oriented polyester film comprising the same. The polyester resin comprises a recurring unit represented by the following formula (A) and a recurring unit represented by the following formula (B) as the main constituents: —O—C(O)—R1—C(O)—O—R2—O—  (A) —O—C(O)—R3—C(O)—O—R2—O—  (B) (R′ is a phenylene group or naphthalenediyl group, R2 is an alkylene group having 2 to 4 carbon atoms or cyclohexylene group, and R3 is 6,6′-(alkylenedioxy)di-2-naphthoic acid), wherein the content of the recurring unit (B) is not less than 5 mol % and less than 50 mol %, and the ratio of adjacent recurring units (A) and (B) is less than 0.9 based on a value obtained by doubling the product of the content of the recurring unit (A) and the content of the recurring unit (B).