The inventor of the present invention has made eager researches to aim a complete-cycle type recycling of polylactic acid and overthrown common knowledge about 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.