The invention relates to a method for hydrolyzing a plant fiber material and producing and separating a saccharide including glucose. The method of the invention includes a hydrolysis process of using a cluster acid catalyst in a pseudo-molten state to hydrolyze cellulose contained in the plant fiber material and produce glucose. In the hydrolysis process, the cluster acid catalyst and a first amount of the plant fiber material that increases a viscosity of the cluster acid catalyst in a pseudo-molten state when added to the cluster acid catalyst in a pseudo-molten state are heated and mixed, and a second amount of the plant fiber material is then further added when the decrease in viscosity of the heated mixture occurs.