A monosaccharide of galacturonic acid and its oligomers were produced by hydrolysis of poly(galacturonic acid)s with water in the absence of any additives in a semibatch flow reactor over the temperature range from 453 to 533 K at 10 MPa. A solid sample of poly(galacturonic acid) was fixed in the reactor, and the residence times of water in the reactor were estimated to be less than a few seconds. At 493 K and a heating time of 2 min, 79 wt % of the initial sample on the carbon weight basis was converted into water-soluble components, which consisted of monomer and oligomers having various degrees of polymerization (DP) and were enzymatically hydrolyzable into mono(galacturonic acid), the dimer, and the trimer. The maximum yields of monomer and oligomers with DP = 2-10 were 11.4 and 22.1 wt %, respectively, on the carbon weight basis of the initial sample at 533 K, 10 MPa, and a heating time of 2 min. The yield of each product was correlated with the content of organic carbon in the eluted product solution. The formation of the further decomposed products such as furfural and gaseous products was insignificant under the hydrolysis conditions studied.