it is found that the dehydration of glucose to anhydroglucose, such as 1,6-anhydro-beta-D-glucopyranose and 1,6-anhydro-beta-D-glucofuranose, proceeds very effectively in hydrophobic ionic liquids, for example ionic liquid N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethanesulfonyl)amide, without any catalysts at high temperature. On the other hand, addition of a small amount of an ionic liquid with chloride anion reduces the yield of the anhydroglucose significantly.