A novel polymerization catalyst for polyesters, which does not contain any
germanium or antimony compound as the main component; polyesters produced
with the catalyst; and a process for producing polyesters. This
polymerization catalyst is excellent in catalytic activity, little causes
thermal degradation of polyesters in melt molding even when neither
deactivated nor removed, and can give thermally stable polyesters which
little generate foreign matter and are excellent in transparency and
color. The polymerization catalyst is one which contains as the first
metal-containing component at least one member selected from the group
consisting of aluminum and aluminum compounds and which gives
polyethylene terephthalate (PET) having a thermal stability parameter
(TS) satisfying the relationship: (1) TS<0.3.