In a reaction chamber (20), a used alkaline permanganate etching solution (12) is accommodated and an alkaline earth hydroxide (14) such as calcium hydroxide is added in the reaction chamber, a liquid inside of the reaction chamber is agitated, the liquid is exhausted from a side portion or a top portion of the reaction chamber through a filter (28), a precipitate (26) adhered to the filter is scraped off, and a precipitate containing a hardly soluble or insoluble matter incapable of passing through the filter and accumulated on a bottom portion of the reaction chamber is exhausted from the reaction chamber.