TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The carbonyl-containing repeating unit (A) is of formula (1). The carbonyl-containing repeating unit (B) is of formula (2 or 3). The content of repeating unit (A) and repeating unit (2) contained in the dicarboxylic acid component is 10-40 mol% and 90-60 mol%, respectively. The content of repeating unit (A) and repeating unit (3) contained in the dicarboxylic acid component is 5-45 mol% and 95-55 mol%, respectively. A composition contains polyester, and particles (P') (0.01-50 weight%) with average particle diameter of 0.05- 5 mu m and volume shape factor of 0.4- pi /6. Preferred Properties: The polyester has intrinsic viscosity of 0.4-3 dl/g at 35 degrees C, measured using a mixed solvent of p-chlorophenol and 1,1,2,2-tetrachloroethane in weight ratio of 40:60, melting point of 200-260 degrees C, and 0-1 endothermic peaks at 120-220 degrees C when cooled at 10 degrees C/minute. A film formed using the polyester, has Young's modulus (Y') of 4.5 GPa or more (preferably 6 GPa or more), along one direction in film plane direction, 5 GPa or more, along both directions in film plane directions, temperature expansion coefficient (( alpha )t) of 14x 10-6/ degrees C or less (preferably 10x 10-6/ degrees C or less), along one direction (preferably width direction) in film plane direction and humidity expansion coefficient (( alpha )h) of 1-7x 10-6/%RH, along one direction in film plane direction. The Young's modulus (Y') and humidity expansion coefficient (( alpha )h) satisfy a relation: (( alpha )h) less than or equal to (1.2Y'+17).    TECHNOLOGY FOCUS - POLYMERS - Preferred Particles: The particles (P') are chosen from silica particles and organic polymer particles, or silicone-resin particles and crosslinked polystyrene particles. EXAMPLE - Dimethyl terephthalate, 6,6'-(ethylenedioxy)di-2-naphthoic acid and ethylene glycol were esterified in presence of titanium tetrabutoxide, and polycondensed to obtain polyester having intrinsic viscosity of 0.73 dl/g, melting point of 233 degrees C, glass transition temperature of 91 degrees C, 65 mol% of terephthalic acid component and 35 mol% of 6,6'-(ethylenedioxy)di-2-naphthoic acid. A film formed using the obtained polyester, had excellent dimensional stability and mechanical strength.