To sort plastic pieces in high precision by their kinds by molding a part of a counter electrode excluding an effective range forming a static electric field opposite to a drum electrode from a resin material, installing an air supply means for ejecting air to remove adherent plastic pieces, and ejecting air intermittently.

A high voltage electrode (counter electrode) 8 is composed of a plate part 15 and a curved surface part 16 which is curved opposite to a metal drum electrode 6 in both side fringes of the plate part 15. The radius(r) of the part 16 is at least 1/2 of the distance L between the electrode 6 and the electrode 8. A side fringe part 17 excluding a surface facing the electrode 6 is insulated by being molded from a resin material. Moreover, the electrode 8 is supported by an electrode holder 18, and vent holes 19 for ejecting air parallel to the electrode 8 are formed in the holder 18. A vent pipe 20 for supplying air is connected to the vent holes 19.