Highly negatively charged heteropolyacids (HPAs), in particular H5BW12O40, efficiently promoted saccharification of crystalline cellulose into water-soluble saccharides in concentrated aqueous solutions (e.g., 82 % total yield and 77 % glucose yield, based on cellulose with a 0.7 M H5BW12O40 solution); the performance was much better than those of previously reported systems with commonly utilized mineral acids (e.g., H2SO4 and HCl) and HPAs (e.g., H3PW12O40 and H4SiW12O40). Besides crystalline cellulose, the present system was applicable to the selective transformation of cellobiose, starch, and xylan to the corresponding monosaccharides such as glucose and xylose. In addition, one-pot synthesis of levulinic acid and sorbitol directly from cellulose was realized by using concentrated HPA solutions. The present system, concentrated aqueous solutions of highly negatively charged HPAs, was further applicable to saccharification of natural (non-purified) lignocellulose biomass, such as “rice plant straw”, “oil palm empty fruit bunch (palm EFB) fiber”, and “Japanese cedar sawdust”, giving a mixture of the corresponding water-soluble saccharides, such as glucose (main product), galactose, mannose, xylose, arabinose, and cellobiose, in high yields (≥77 % total yields of saccharides based on holocellulose). Separation of the saccharides and H5BW12O40 was easy, and the retrieved H5BW12O40 could repeatedly be used without appreciable loss of the high performance.