conventional enzymatic degradation of polylactic acid and found out that polylactic acid can be depolymerized into an oligomer with an enzyme and further the generated oligomer is a re-polymerizable oligomer mixture and can easily be re-polymerized into polylactic acid. The generated oligomer is a mixture composed of various low molecular weight compounds. The re-polymerization of this mixture makes it possible to yield polylactic acid. The re-polymerization may be chemically synthetic polymerization, that is, polymerization using a catalyst besides polymerization using a hydrolase.