A method of treating an aqueous solution containing quantum dots, the method includes: a dissolution liquid preparation step of dissolving a quaternary ammonium salt and a potassium salt in an aqueous solution containing quantum dots to prepare a dissolution liquid; a phase separation step of heating the dissolution liquid to separate the dissolution liquid into two phases, which are a quaternary ammonium salt phase containing a large amount of the quaternary ammonium salt and a potassium salt phase containing a large amount of the potassium salt, and also to cause the quantum dots to be included in either one of the quaternary ammonium salt phase and the potassium salt phase; and a fractionation step of separating the quaternary ammonium salt phase and the potassium salt phase by fractionation.