NOVELTY - A polyester film contains a polyester layer (P) which consists of polyester composition containing crystalline polyester resin containing 90 mol% or more of 1,4-cyclohexylene dimethylene terephthalate units and additive (crystallization promoter) which has the crystallization enhancement effect with respect to the resin, and has crystallization heat-of-fusion of 20 J/g or more at 1st RUN measured using differential scanning calorimetry, half value width of 18 degrees C or less with respect to crystal melting peak height, and bubble content rate of 3 area% or less.    USE - Polyester film is used for back sheet of solar cell and reflecting plate of liquid-emitting diode (both claimed). Uses include but are not limited to flexible printed wiring board, membrane switch, planar heating element, flat cable, magnetic recording material, packing material, capacitor, motor-vehicle component, thermal-transfer material, etc.    ADVANTAGE - The polyester film has high heat and moisture resistance for long period of time, dimensional stability and mechanical characteristics.    DETAILED DESCRIPTION - A polyester film contains a polyester layer (P) which consists of polyester composition containing crystalline polyester resin containing 90 mol% or more of 1,4-cyclohexylene dimethylene terephthalate units and additive (crystallization promoter) which has the crystallization enhancement effect with respect to the resin, and has crystallization heat-of-fusion of 20 J/g or more at 1st RUN measured using differential scanning calorimetry, half value width of 18 degrees C or less with respect to crystal melting peak height, and bubble content rate of 3 area% or less. The content of crystallization promoter is 0.5-30 %mass with respect to gross mass of polyester layer. The crystalline polyester resin which does not contain the crystallization promoter and the composition of the crystalline polyester resin containing the crystallization promoter are heated with temperature increase rate of 20 degrees C/minute based on JIS K7122 (1987). The crystalline polyester resin is heated with temperature increase rate of 20 degrees C/minute (1st RUN) from 25 degrees C to 40 degrees C of melting point and then allowed to standstill for 5 minutes. The resin is cooled to 25 degrees C or less. Further the resin is heated from room temperature to 40 degrees C with temperature increase rate of 20 degrees C/minute to 40 degrees C (2nd RUN). The peak top of the crystallization peak in 2nd RUN appears in a low temperature side compared with that in which the direction of the composition of crystalline polyester resin containing the crystallization promoter does not contain the crystallization promoter when compared with differential-scanning-calorimetry chart.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic sectional view of solar cell. (Drawing includes non-English language text)    Solar cell back sheet (1)    Transparent filler (2)    Electricity-generating element (3)    Transparent substrate (4)