To improve sorting treatment ability by electrifying charges necessary for electrostatic separation in spite of a short electrification time.

The plastic pieces prior to triboelectrification is heated by a heater 1 and thermal excitation is induced during the triboelectrification, by which contact electrification time is shortened in separating the plastic pieces by material by electrifying the plastic pieces by the triboelectrification device 2 to throw a static electric field between electrodes 6 and 7 and changing the falling loci in the static electric field 10 by the charges possessed by the plastic pieces and the polarities thereof.