Cellulose contained in plant fiber material is hydrolyzed with the use of a pseudo-molten cluster acid as a hydrolysis catalyst to produce saccharide, most of which is glucose. After the glucose is produced, the saccharide is precipitated with the use of an organic solvent, and the saccharide including a solidified saccharide during the hydrolysis and the precipitated saccharide is separated from residues and the cluster acid.