To reduce a use amount of a washing liquid by a method wherein, when target constituents are separated by using a washing liquid or the like, the separation is executed by a separation means excepting a film treatment while the washing liquid is added to a raw material, and the washing liquids of different target constituent concentrations which have been separately prepared are processed by a multi-stage countercurrent operation in the order from higher target constituent concentration.

In the case where a soluble substance of an incineration ash is removed in order to recycle the incineration ash by an incineration process of a refuse, a storage tank A for a raw material is installed, and a removing treatment of the soluble substance in the incineration ash is executed by using a super-decanter as a solid-liquid separator G. Target constituents in the raw material are soluble substances required for being removed, and water is used as a washing liquid. Then, for the raw material, though adjustment of a slurry liquid of the incineration ash is performed as a pretreatment, a specific amount of the powdery incineration ash in a hopper B is supplied to a repulper J together with a washing liquid I of a medium concentration or those of II to IV to make the slurry liquid in this case. The slurry liquid is circulated by a route passing through the solid-liquid separator G from the storing tank A, and the target constituents are taken out by separation.