The object is to produce a polysaccharide and/or a monosaccharide efficiently by hydrolyzing a different polysaccharide efficiently. The hydrolysis of a polysaccharide is an important means for producing a monosaccharide that can be used as a starting material for the production of ethanol, the solubilization of a water-insoluble polysaccharide, and the production of a useful water-soluble low-polymeric saccharide or the like. For achieving the object, a polysaccharide to be hydrolyzed is reacted with water in the presence of a carbonaceous material having sulfonic acid group therein to cause the hydrolysis of the polysaccharide to be hydrolyzed, thereby producing a other polysaccharide and/or a monosaccharide.