PURPOSE: To maintain refining capacity of high accuracy safely over a long period of time and reduce the times of maintenance and inspection by disposing dust collectors between oppositely disposed electrodes in a manner as not to contact said electrodes.

CONSTITUTION: Plural ones of electrodes 10 are oppositely disposed at every after required pitch. Plural dust collectors 11 are disposed between pairs of oppositely facing electrodes 10 in parallel each other with the electrodes 10, and clearances 12 are maintained between the electrodes 10 and the collectors 11 as well as between the collectors 11 and themselves. In this state, the collectors 11 are fixed at their one end and other end by supports 13 of insulation characteristic provided on the outer side parts of the clearances 12. Since this makes it possible to surely prevent the generation of leak current between the electrodes 12 or shortcircuiting therebetween, fires, explosions and the like are prevented. In addition, there is no need for operating the device on low voltage, and therefore the efficiency of dust collection is improved.