A polymerization method comprises, prior to polymerization of a monomer of the cyclic dimer of alpha-hydroxycarboxylic acid, a first mixture of a polymerization catalyst by bypassing a part of the monomer of the cyclic dimer is prepared. Then, the first mixture is mixed with a dominant amount of the monomer introduced into a polymerization reactor. The monomer of the cyclic dimer in the polymerization reactor is conducted in the presence of a polymerization initiator.