PURPOSE: To efficiently recover useful metals from dust by removing long-sized objects from the dust and selecting the dust to light objects and heavy objects by using a heavy liquid after washing of the dust, etc., with water, then separating the light objects to nonferrous metals and nonmetals after washing the heavy liquid with water.

CONSTITUTION: A dust treating device 10 first selects the long-sized objects from the dust by a long-sized object selector 11 and washes the fine dust and oil-components sticking to the selected sizes of prescribed sizes of the dust with a prewashing machine 12. The washed dust is selected to the light objects 14 and the heavy objects 15 by the heavy liquid 13 in a heavy liquid selector 16. Next, the heavy liquid 30 adhered to the light objects 14 and the heavy objects 15 is washed off by water washing machines 17, 18 and the magnetic powder in the heavy liquid 13 is attracted and separated by a magnetic selector 19. In succession, the impurities in the waste water of this magnetic selector 19 are carried to a water tank 21 by a discharge conveyor 20. The attracted magnetic powder is demagnetized by a demagnetizing machine 22. The heavy liquid 13 contg. the demagnetized magnetic powder is thereafter charged into a heavy liquid tank 24.