A biaxially oriented polyester film for use in a capacitor having improved break down strength, which polyester having an acid component at least 90 % by mole of which is terephthalic acid and a glycol component at least 97 % by mole of which is 1,4-cyclohexanedimethanol, in which film, an extracted amount with chloroform is not more than 1.3 % by weight per 24 hours.
