NOVELTY - A biaxial-oriented polyester film is polyester film having 50 or more layers including layer (A) containing polyester (p1), and layer (B') containing polyester (p2), which are sequentially laminated along thickness direction. Polyester (p1) has crystal melting heat rate of 20 J/g or more and less than 60 J/g at temperature increase rate of 20 degrees C/minute measured by differential scanning calorimetry method. Polyester (p2) has crystal melting heat rate of less than 0.01 J/g at temperature increase rate of 20 degrees C/minute measured by differential scanning calorimetry method.    USE - Biaxial-oriented polyester film is used for pharmaceutical packaging applications and exterior of battery (all claimed) e.g. lithium secondary battery.    ADVANTAGE - The biaxial-oriented polyester film has excellent moldability and curvature-proof property. The battery using the biaxial-oriented polyester film has excellent capacity.    DETAILED DESCRIPTION - A biaxial-oriented polyester film is polyester film having 50 or more layers including layer (A) containing polyester (p1), and layer (B') containing polyester (p2), which are sequentially laminated along thickness direction. Polyester (p1) has crystal melting heat rate of 20 J/g or more and less than 60 J/g at temperature increase rate of 20 degrees C/minute measured by differential scanning calorimetry method. Polyester (p2) has crystal melting heat rate of less than 0.01 J/g at temperature increase rate of 20 degrees C/minute measured by differential scanning calorimetry method, in which lamination ratio of layer (A)/layer (B') is 1-6. Or polyester (p2) has crystal melting heat rate of 20 J/g or more and less than 60 J/g at temperature increase rate of 20 degrees C/minute measured by differential scanning calorimetry method, in which lamination ratio of layer (A)/layer (B') is 0.3-5. The biaxial-oriented polyester film has stress ratio (( sigma 1)/( sigma 2)) of 1.5 or more measured as ratio of tensile stress ( sigma 1) at the time of elongation rate of 50% and tensile stress ( sigma 2) in a yield point, and tensile stress ( sigma 2) of less than 100 MPa, and elastic modulus of less than 4000 MPa, measured by stress-elongation curve in tension test.