PURPOSE: To separate and recover dust into plural grain size distributions by separating the dust having ≥2 kinds of grain size by introducing gas into a dust collector and further introducing a secondary dust collecting filter.

CONSTITUTION: The gas G generated in a vertical type furnace is introduced into an inertia dust collector 6 in the dust collecting device and flowed in louver type gas flowing passages 7 arranged at plural steps and the gas G containing the dust is divided with the inertia. The divided gases G are flowed into each secondary dust collecting filter 9-1W9-3. The gas G having the dust of the min. grain size is passed through the upper part passage (a) and in order the gases G having the dust of larger grain size are passed through the middle passage (b) and the lower part passage (c), respectively. These dusts having each different grain size are deposited into each dust hopper 10-0W10-3. The divided gas G separating each grain size of the dust is joined with a pipe 11. By this method, the dust to be possible to reutilize can be increased.