Polybutylene terephthalate having good
heat stability and excellent
hydrolysis resistance is continuously produced in a series of a first reactor for reacting an aromatic
dicarboxylic acid comprising
terephthalic acid as a main ingredient or a derivative thereof with a glycol comprising 1,4-
butanediol as a main ingredient, thereby producing an
oligomer with an average
degree of polymerization of 2.2 to 5, a second reactor for polycondensating the
oligomer from the first reactor, thereby preparing a low
polymerization product with an average
degree of polymerization of 25 to 40, and a third reactor for further polycondensating the low
polymerization product from the second reactor, thereby producing a high molecular weight
polyester with an average
degree of polymerization of 70 to 130, or followed by a fourth reactor for further polycondensing the
polyester from the third reactor to an average degree of
polymerization of 150 to 200, thereby producing a high molecular weight
polyester. Another third reactor or a plurality of third reactors can be provided in parallel to the third reactor, thereby producing different kinds of
polybutylene phthalate with different degrees of polymerization from that produced in the main line of the third and fourth reactors or adjusting operating conditions of each of a plurality of the third reactors to increase kinds, precise quality.