TECHNOLOGY FOCUS - POLYMERS - Preferred Structure: The polyester resin contains a structure of formula (III) derived from bisphenol, and structures of formulae (IV,V) derived from divalent carboxylic acid. (a+b)/(a+b+c) is 0.2-0.9, where a is the content ratio (mol%) of structure (I), b is the content ratio (mol%) of structure (II), and c is the content ratio (mol%) of structure (II).R31-R38=H or substituent;X=optionally substituted linking group or may be a portion of ring structure, and, in such case, may be bonded with at least one of R31-R34 to form a ring structure;R41,R51,R52=substituent;andm,n,k=0-3.Preferred Properties: The polyester resin has a glass transition temperature of 170-270 degrees C. A film formed using the polyester resin has a coefficient of linear thermal expansion of 40 ppm/K or less. A gas barrier layer and a transparent electroconductive layer are provided in the film. Preferred Definitions:    R15-R18,R21-R26=fluoro, chloro, bromo, 1-4C alkyl, phenyl, or methoxy.    EXAMPLE - Into a 300 ml flask, (in g) 3,3',5,5'-tetramethyl-4,4'-bishydroxybiphenyl (10.46), 3,3'-dimethyl-4,4'-bishydroxybiphenyl (4.68), bisphenol A (9.96), sodium hydrosulfite (360 mg), tetra-n-butylammonium chloride (1600 mg), methylene chloride (390 ml), and distilled water (450 ml) were added, stirred and dissolved under nitrogen flow. To the solution, solution of terephthaloyl chloride (17.8) and 2,6-naphthalene dicarboxylic acid chloride (5.36) dissolved in methylene chloride (180 ml) was added. Further, a mixed liquid of 2 mol/L aqueous sodium hydroxide solution (114 ml) and water (30 ml) were dripped at 15-20 degrees C over 1 hour. After dripping, the liquid was stirred for 3 hours, and then the reaction liquid was transferred to a flask, to which acetic acid (1.8 ml) and ethyl acetate (1800 ml) were added slowly. After filtering, the obtained polymer powder was washed sequentially with ethyl acetate (1 L), water (1 L), and methanol (1 L) and dried, to obtain a polymer (36.4) having a weight average molecular weight of 105000 by polystyrene conversion. The polymer was dissolved in methylene chloride and flow-cast over a glass plate, and dried to obtain a film having a thickness of 100 mu m. The film had a glass transition temperature of 257 degrees C, a thermal expansion coefficient of 10 ppm/K, and excellent transparency.