To prevent the ignition/explosion of plastic pieces in the classification of the pieces.

In a plastic classifier in which charged plastic pieces 3 are passed between a metal drum 1 and an electrode plate 2 to be classified, a limiting resistance 20 for limiting the energy of a spark 10 is inserted between a high-voltage direct current power source 5 and the electrode plate 2, and the resistance R of the limiting resistance 20 is set up to meet R> Rp0/{(E×Rp0×vd)/[Va2×(Φs+Φp)-1} wherein Rp0 is the resistance between the drum 1 and the electrode plate 2 with no spark 10 generated; Va is a voltage between the drum 1 and the electrode plate 2; E is the minimum ignition energy of the plastic pieces 3; vd is the circumferential velocity of the drum; Φs is the diameter of the spark 10; Φp is the diameter of the plastic pieces.