PROBLEM TO BE SOLVED: To provide a solubilization treatment method for hardly soluble substances in which hardly soluble substances can be chemically transformed into soluble substances while a high level of safety is maintained and the risk of corrosion of structural materials is avoided.SOLUTION: A solubilization treatment method for hardly soluble substances in which hardly soluble substances are chemically transformed into soluble substances, generates chlorine ions and chlorine radicals by generating an atmospheric-pressure nonequilibrium plasma by mixing a carrier gas with a chlorine compound, and chemically transforms a hardly soluble substance into a chloride that is a soluble substance by reacting the generated chlorine ions and chlorine radicals with the hardly soluble substance. The invention also includes a dissolution method for hardly soluble substances in which the chloride that has been chemically transformed using the solubilization treatment method for hardly soluble substances is dissolved
