To provide a polyester film for a gas barrier which shows outstanding working properties and gas barrier properties.

This polyester film for a gas barrier has at least, two layers, i.e., outer layers A, B. A gas barrier layer is formed on the surface 'A' face of the layer A of a laminated base film which is formed as a composite coat by a melt extrusion process. In addition, ARa<BRa, ARa≤0.05 μm, 0.02 μm≤Bra≤0.3 μm are satisfied when the face roughness of the 'A' face is given as ARa and the face roughness of the surface 'B' face of the layer B as BRa. Further, the extraction level of oligomer from the surface layer of the layer A is 50 mg/m2 or less.