PURPOSE: To treat plastics without requiring the accumulation of refuse by perfectly separating plastics different in compsn. and having various densities from a plastic mixture.

CONSTITUTION: A riser pipe 52 into which a homongenous suspension of a separation medium and plastic particles is sent is inserted in a central portion 51 and extends up to the upwardly turned leading end of a conical portion 53 positioned at a lower position and the downwardly turned leading end of a conical portion 54 positioned at an upper position is opposed to the conical portion 53. Both conical portions 53, 54 form a double cone. The suspension to be separated flows out laterally, that is, flows out to spaces 55a, 55b expanded between mutually opposed conical surfaces while the flow velocity of plastic particles decreases. Plastics are separated into a floating portion and a sedimenting portion at the cylindrical central portion 51 and both portions are discharged from the central portion through conduits 56, 57. By this constitution, pure plastics can be regenerated without preliminarily classifying plastics.