To provide an aromatic copolyester, which is obtained by copolymerizing a 6,6'-(alkylenedioxy)di-2-naphthoic acid component and has few surface defects in formation into a film, and a biaxially oriented film using the aromatic copolyester.

The aromatic copolyester comprises an aromatic dicarboxylic acid component and an alkylene glycol component, and on the basis of the whole acid components, a content of a 6,6'-(alkylenedioxy)di-2-naphthoic acid (ANA) component is 5-80 mol%. When molding the aromatic copolyester into a sheet with thickness of 5 m, the number of surface high projection foreign matter containing sodium elements or potassium elements and sulfur elements is not more than 10 per 100 cm2. The biaxially oriented film using the aromatic copolyester is also provided.