To conduct separation of waste plastic chips with high reliability at the electrostatic separation unit of a plastic sorter by adding a specified amt. of a triboelectrification assistant to the plastic chips of different kinds to give each kind of plastic chip a different polarity and amount of charge, to efficiently recover a specified kind of plastic chips and to repeatedly use the assistant by leaving behind the added assistant in the sorter.

When the quantity of chips of specified plastic 1 to be recovered is small, the chips 1 are added into a triboelectrification device 3 as a triboelectrification assistant 21 and agitated to prevent the plastic chips 1 of different kinds (e.g. PVC and PE) from being charged to the same polarity. Subsequently, a specified amount of triboelectric charge is imparted to ensure reliable separation of chips in the electrostatic separation unit, and the specified plastic chips 1 are recovered.