A polyester resin prepared by polycondensing a dicarboxylic acid component containing terephthalic acid or derivative thereof with a diol component containing ethylene glycol through esterification or transesterification, thereby producing a melt polymerization polymer and subjecting the melt polymerization polymer to solid-phase polycondensation, wherein (1) the melt polymerization polymer contains comonomer units other than terephthalic acid units and ethylene glycol units in an amount of 5.5 or less, and whose intrinsic viscosity ranges from 0.08 dL/g to 0.50 dL/g and which has an apparent crystallite size smaller than 9 nm, and wherein (2) the melt polymerization polymer is subjected to solid-phase polycondensation at a temperature which is higher by at least 140.degree. C. than the glass transition temperature of the melt polymerization polymer and is not higher than the melting point of the polymer to thereby obtain a polyester resin having an apparent crystallite size smaller than 9 nm.