To provide an aqueous solution in which a slightly-soluble inorganic substance or the slightly-soluble inorganic substance and an organic component-containing substance are dissolved and to provide a method for preparing the aqueous solution.

When the aqueous solution containing the slightly-soluble inorganic substance is prepared, a material containing the slightly-soluble inorganic substance is decomposed oxidatively under pressure by using a fluorine-containing acid and a strong acid. When the aqueous solution containing the slightly-soluble inorganic substance and the organic component-containing substance is prepared, another material containing the slightly-soluble inorganic substance and the organic component-containing substance is decomposed oxidatively under pressure by using the fluorine-containing acid, the strong acid and an oxidative substance to be added additionally.