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.