TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The polyester film further comprises 1-50% compatibilizing agent with respect to cyclohexanedimethanol-type polyester. Two or more peaks of a crystal melting heat appear before the addition of compatibilizing agent. 70% or more Structures derived from cyclohexyl dimethanol are structures derived from cyclohexyl dimethanol of trans-isomer.    TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The compatibilizing agent is hydrogenation diene-type copolymer (P), polymer (Q) containing olefin-type monomeric unit and/or an aromatic vinyl type monomeric unit (Q1), and monomeric unit (Q2) having groups comprising acid anhydride group, epoxy group, carboxyl group, carboxylate ester group, amino group, nitrile group, hydroxyl group, oxazoline group, amide bond, and ester bond, or, at least polymer block mainly comprising an aromatic vinyl compound, polymer block mainly comprising conjugated diene compound, and hydrogenated polymer containing block copolymer containing an amino group. The amount of terminal carboxylic acid in polyester film is 5-20 eq/ton. Preferred Process: The layer containing cyclohexanedimethanol-type polyester is obtained by adding 10-1000 ppm highly crystalline microparticles of polyester with respect to cyclohexanedimethanol-type polyester, and forming into film by melt-extrusion by giving the difference of 10-150% in the residence time. Preferred Properties: The maximum stress retention of polyester film before and after heating the polyester film at 180 degrees C and relative humidity of 0% for 120 hours is preferably 10-50%. The direction in which maximum stress retention before and after heating with 180 degrees C and relative humidity of 0% for 120 hours becomes the largest is 5-35% more than the maximum stress retention in the direction orthogonal to the direction to become the largest. The direction in which maximum stress retention before and after heating with at 180 degrees C and relative humidity of 0% for 120 hours becomes the largest is direction orthogonal to a film transport direction at the time of film forming. The ratio (A/B) of value (A) obtained by dividing heat-absorption amount of the endothermic peak by heat-absorption amount of the largest endothermic peak and value (B) obtained by dividing heat-absorption amount of endothermic peak by heat-absorption amount of largest endothermic peak after adding compatibilizing agent, is 1.05 or more. The maximum stress of polyester film after heating at 180 degrees C and relative humidity of 0% for 120 hours is 70-150 MPa. The direction orthogonal to the direction having largest elongation retention is 50-95% more than the direction having largest elongation retention before and after heating at 120 degrees C and at relative humidity of 100% for 100 hours. The direction having largest elongation retention before and after heating at 120 degrees C and relative humidity of 100% for 100 hours is direction orthogonal to film transport direction at the time of film forming. The in-plane distribution of largest elongation retention before and after heating at 120 degrees C and at relative humidity of 100% for 100 hours is 10-50 %. The crystallinity degree of highly crystalline microparticles of polyester is 3-30% more than crystallinity degree of cyclohexanedimethanol-type polyester used as raw material pellet. The thickness of both ends at the time of extrusion of the melt containing the cyclohexanedimethanol-type polyester is 0.5-10% rather than the thickness of a center portion.