A feed tray (22) for guiding tribo electrified plastic pieces (2) to a drum electrode (6) is provided with a vibrator (23), thereby making it possible to vibrate the plastic pieces (2) to form a thin uniform layer, and to reliably apply appropriate image force and centrifugal force to the plastic pieces (2) fed to the drum electrode (6); thus, the plastic pieces (2) can be separated according to kind with high precision.