To conduct floatation of synthetic resins which cannot be heretofore separated by being simplified in the regulation of best floatation condition without necessity of sterilizing reagent with less type of chemicals.

The surface treatment of synthetic resin particle in terms of the change of wet stage of the particles responsive to the type of the synthetic resin is conducted by a plasma treating of particle mixture and a physically drying system by storage continued thereto out of a flotation machine before flotation. The plasma treatment of the mixture is conducted under the pressure of about 10Pa in the presence of oxygen adapted for process gas of microwave excitation in a sealed treating chamber, a for example, for 5 to 10sec in loose bulk stock. Instead, the plasma treatment of the mixture is conducted while continuously conveying the mixture to be treated to the treating area in the presence of air in the opened atmosphere.