Abstract: A polyester formed by adding compounds of the following formulae I and II to a reaction product of terephthalic acid or a dicarboxylic acid mixture mainly composed of terephthalic acid or an ester-forming derivative thereof with at least one glycol or an ester-forming derivative thereof, having a melt electrical resistance of not higher than 1600.times.10.sup.6 .OMEGA. and a softening point higher than the softening point derived from Flory's equation for the melting temperature of random copolymers but lower than the softening point of a polyester obtained by adding a compound of formula I alone: ##STR1## wherein each R independently represents a hydrogen atom or an alkyl group having 1 or 2 carbon atoms.