A biaxially oriented saturated polyester film made by a biaxial orientation method, includes a heat-seal side formed in at least one of two sides of the film, the heat-seal side being made by applying a low-temperature plasma treatment to a film surface so that the film surface has a composition ratio of the number of oxygen atoms to the number of carbon atoms, the composition ratio being not less than 105% and not more than 115% of a theoretical value and so that the heat-seal side is heat sealable at a heating temperature ranging from 100.degree. C. to 200.degree. C.