PURPOSE: To facilitate the classification of a flocculated powdery fluid by dispersing the same in a monodisperse state, by generating a plasma ion due to a creeping streamer by applying high voltage while exposing the flocculated powdery fluid to the plasma ion generated region.

CONSTITUTION: Rings 3, 4 made of, for example, glass are provided between pipes 1, 2 each made of a conductive metal, for example, brass so as to interpose a ring 5 made of a conductive metal therebetween. An AC power source 6 is used and both terminals of a high voltage generating circuit 7 are connected to the rings 3, 4 through the pipes 1, 2 to generate a creeping streamer as shown in the intermediate part between the pipes 1, 2 and the rings 3, 4. Whereby, a plasma ion is formed richly in the vicinity of the internal hole between the rings 3, 4. A flocculated substance receives dispersing action when passed through the internal hole 8 rich in the plasma ion to be brought to a monodisperse state and the classification thereof becomes easy.