<p>The present invention relates to a process for decomposing a polymer into a monomer or oligomer by hydrolysis with sub- or supercritical water. The process of the present invention is characterized in that at least a part of the polymer is a polymer comprising a constitutional unit derived from an organic acid in the molecular structure, and the polymer being contacted with sub- or supercritical water in the presence of a water-insoluble base.</p>
