To sort specific plastic chips from plastic chips without fail by adding a frictional electrification assistant material in an amount depending on a number obtained by dividing the total surface area of plastic chips satisfying a specific formula by the surface area of the frictional electrification assistant material.

The weight of specific plastic chips 1a (Wt) in a frictional electrification apparatus 3, the weight of a frictional electrification assistant material added (Wm), and the total weight of plastic chips 1 (W) in the frictional electrification apparatus 3 are set according to the equation: (Wt+Wm)/W =0.1-0.9. The amount of the frictional electrification assistant material added is set depending on the number obtained by dividing the total surface area of plastic chips 1 satisfying the equation by a specified surface area of the frictional electrification assistant material. Plastic chips 1 comprising a mixture of a plurality of kinds of crushed plastic chips are charged into the frictional electrification apparatus 3 and stirred to cause frictional electrification of plastic chips 1 with charges corresponding to the kinds of plastic chips, and then, plastic chips 1 are electrostatically separated from each other and sorted in an electrostatic separation section G.