A solution comprising an added water-soluble xylan, wherein the water-soluble xylan is a molecule having six or more xylose residues connected to each other by β-1,4 bonds and is capable of dissolving in water at 20° C. at a concentration of at least 6 mg/mL, a substance, and a solvent, wherein only less than about 10 g of the substance can be dissolved in 1 L of the solvent in the absence of the water-soluble xylan, wherein said substance is a carbon nanotube, wherein the number average degree of polymerization of the main chain of said water-soluble xylan is 6 or more and 5,000 or less, wherein the number-average molecular weight of said water-soluble xylan is 7,000 or more and 1,000,000 or less, wherein said water-soluble xylan is unmodified glucuronoxylan or unmodified arabinoglucuronoxylan.