Polybutylene terephthalate is produced by continuously subjecting
terephthalic acid and 1,4-butanediol to an esterification reaction in the
presence of a titanium compound catalyst using one-stage or plural-stage
esterification reaction vessel(s) and then subjecting the resultant
reaction product to a polycondensation reaction, wherein the
esterification reaction between terephthalic acid and 1,4-butanediol is
conducted in the presence of a titanium compound having a specific
concentration under a specific pressure at a specific molar ratio between
the raw materials. The process avoids undesirable side reactions by
continuously conducting esterification reaction using terephthalic acid
as a raw material, and can minimize loss of raw materials.