A polishing agent regenerating method in which a component of a polished material is removed from polishing agent slurry and a polishing agent is collected and regenerated is shown. The method includes at least, polishing, polishing agent slurry supplying, polishing agent slurry collecting, and sedimenting/separating/concentrating, performed in the above order. In the polishing agent slurry collecting or the sedimenting/separating/concentrating, a K2O density in the polishing agent slurry after dilution by water is performed is to be within a range of 0.002-0.2 mass %.