Process for producing polyester resin

A process for producing a polyester resin which comprises subjecting a dicarboxylic acid component containing terephthalic acid or an ester-forming derivative thereof as the main component to polycondensation with a diol component containing ethylene glycol as the main component through esterification or transesterification to thereby produce a melt polymerization polymer and subjecting the melt polymerization polymer to solid-phase polycondensation, characterized in that:

• (1) a melt polymerization polymer in which the amount of comonomer units other than terephthalic acid units and ethylene glycol units is 5.5 or less, and which has an intrinsic viscosity of from 0.08 dL/g to 0.50 dL/g and an apparent crystallite size smaller than 9 nm is obtained, and
• (2) the melt polymerization polymer is subjected to solid-phase polycondensation at a temperature which is higher by at least 140° 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.