The present invention provides a polishing slurry capable of polishing even high-hardness materials such as silicon carbide and gallium nitride at a high polishing speed. The present invention is a polishing slurry including a slurry containing a manganese oxide particle and a manganate ion for polishing high-hardness materials having a Mohs hardness of 8 or higher. In the present invention, the manganese oxide particle in the slurry is preferably 1.0 mass % or more; the manganese oxide is preferably manganese dioxide; and the manganate ion is preferably permanganate ion. The polishing slurry according to the present invention enables even high-hardness hardly-machinable materials such as silicon carbide and gallium nitride to be polished smoothly at a high speed.