NOVELTY - A polyester material having intrinsic viscosity of 0.7-1 dl/g and terminal carboxy-group concentration of 20 equivalent/ton or less, is melt-extruded to obtain unstretched film. The unstretched film is stretched along longitudinal direction and then stretched along width direction to obtain biaxially-stretched film. The obtained film is subjected to heat-setting process at 170-210 degrees C, and thermal relaxation process at 150-190 degrees C to obtain polyester film (200). The product (XY) of X and Y after thermal relaxation is 15-80.    USE - Polyester film is used for solar cell backsheet for solar cell module (all claimed).    ADVANTAGE - The polyester film has excellent lubricant property, adhesiveness and weather resistance. The solar cell module using the solar cell backsheet including the polyester film has excellent durability.    DETAILED DESCRIPTION - A polyester material having intrinsic viscosity of 0.7-1 dl/g and terminal carboxy-group concentration of 20 equivalent/ton or less, is melt-extruded to obtain unstretched film. The unstretched film is stretched along longitudinal direction and then stretched along width direction to obtain biaxially-stretched film. The obtained film is subjected to heat-setting process at 170-210 degrees C, and thermal relaxation process at 150-190 degrees C to obtain polyester film. The heat-set biaxially-stretched film has ratio (X/Y) of thermal relaxation rate (X%) along width direction and thermal relaxation rate (Y%) along longitudinal direction of 0.6-1.67 at 150-190 degrees C. The product (XY) of thermal relaxation rate (X) and thermal relaxation rate (Y) is 15-80. The polyester film has central-line average surface roughness of 10-60 nm. INDEPENDENT CLAIMS are included for the following:    (1) polyester film, which has intrinsic viscosity of 0.7-1 dl/g, terminal carboxy-group concentration of 20 equivalent/ton or less, density of 1.385-1.395 g/cm3, heat shrinkage (A%) along width direction and heat shrinkage (B%) along longitudinal direction at 150 degrees C for 30 minutes of each 0.6% or less, ratio of heat shrinkage (A) and heat shrinkage (B) of 0.6-1.67, and central-line average surface roughness of 10-60 nm; and    (2) solar cell module, which comprises transparent substrate through which sunlight is injected, solar cell element, and solar cell backsheet.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic structural view of biaxially stretching machine.    Points (P,Q)    Holding component (2a)    Edge of rail (2b)    Annular rails (60a, 6b)    Biaxial stretching machine (100)    Polyester film (200)