There is provided a treatment method of efficiently separating an additive component from a resin component for the purpose of treating and recycling a thermoplastic resin composition containing an additive. The treatment method includes heating and agitating the thermoplastic resin composition containing the additive together with a solvent for dissolving at least part of the additive at a temperature ranging from the glass transition temperature of the thermoplastic resin to the boiling point of the solvent inclusive, and separating and recovering the solvent in a liquid state in which at least part of the additive is dissolved, so that at least part of the additive is separated and removed from the thermoplastic resin composition.