Biaxially oriented polyester film

To provide a biaxially oriented polyester film that has highly smooth surfaces on both sides, thereby suppressing transfer roughness of a surface shape that is transferred onto a release layer after being wound as a roll, and having a smooth release surface. Provided is a biaxially oriented polyester film in which, among values of maximum protrusion heights in each field of view obtained by the following method on one surface (A surface) and the other surface (B surface), when the values of the maximum protrusion heights corresponding to values of the top 5% are respectively defined as Sp5%A (nm) and Sp5%B (nm), Sp5%A is 110 or lower, and Sp5%B is 150 to 1000. (Maximum protrusion height measurement method) Ten objective lenses are used with a scanning-type white light interference microscope (VertScan) to measure 100 fields of view of a surface image in a 561 [mu]m field of view, the maximum protrusion height Sp value in each field of view is obtained, and values corresponding to the top 5% are defined as the