To eliminate the need for sampling and to sufficiently electrify plastic pieces even if the kinds and conditions of the materials to be sorted change.

The plastic sorter for sorting plastic pieces by kinds by feeding the plastic pieces triboelectrified by a triboelectrifying section 1 into an electrostatic field 33 for separation of an electrostatic sorting section 2 and changing the falling loci of the plastic pieces by electrostatic force is constituted by forming a monitor window 20 to an electrifying vessel for agitating and electrifying the plastic pieces color-coded by the kinds. The plastic pieces stuck to the monitor window 20 are picked up by a CCD camera 21 and the kinds of the plastic piece sticking to the monitor window 20 are identified from the image data of the CCD camera 21 by an image processing section 22. Judgment is made by an electrification deciding section 23 that the electrification quantity of the plastic pieces attains a state near the saturated state when one kind of the plastic pieces sticking to the monitor window 20 occupy a prescribed rate or above of the area of the monitor window 20 from the data of the image processing section 22.