PURPOSE: To classify simply with high precision and stability into many grades as the need arises.

CONSTITUTION: Linear cross electrodes 34a, 34b, 34c and linear cross electrodes 34x, 34y, 34z are arranged to cross at right angles correspondingly. When low voltage with two or more phases is applied only to the linear cross electrodes 34a, 34b, 34c, the lightest powder moves to be extracted. When higher voltage with two or more phases is applied to the linear cross electrodes 34a, 34b, 34c and the linear cross electrodes 34x, 34y, 34z, the second lightest powder moves in the direction other than that of the lightest powder to be extracted. When a still higher voltage is applied with two or more phases is applied to only the linear cross electrodes 34x, 34y, 34z, the third lightest powder moves in the direction other than those of the previous powder to be extracted.