To provide a method for producing a high-grade recycled Ni sludge from which Ni can be economically recovered, by separating Ni sludge from a washing water originating in a Ni-plating process.

This production method includes: providing areas in which a plurality of plating lines that are formed of a Ni-plating process or a composite process of Ni plating and other metal plating are arranged in parallel; also providing a waste water treatment line for producing sludge from the washing water in the Ni-plating process in the same area or in a place adjacent to the same area; pouring an organic polymer flocculant into a flocculation tank; settling the agglomerate in a settling tank to separate it; extracting the settled sludge from the bottom; returning the sludge back to a pH control tank at least one time to make the sludge recycled; extracting the settled sludge again and sending it to a sludge tank in the next concentration step; and subsequently extracting the sludge while storing it in the sludge tank and sending it to a hydro extractor to obtain the sludge. The sludge becomes a valuable that contains a high concentration of Ni, from which Ni can be economically recovered by a processor.