[Object] When rare earth magnets are plated, components of the rare earth magnets are dissolved in the plating solution, causing plating defects. Thus, an easy method for removing rare earth impurities has been necessary.[Means for Solution] A nickel-electroplating solution containing rare earth impurities is kept at 60° C. or higher for a predetermined period of time to precipitate rare earth impurities for separation by sedimentation or filtration. Rare earth impurities can be precipitated further efficiently by adding precipitate to the nickel-electroplating solution, or by concentrating the nickel-electroplating solution by heating.