Disclosed is a polishing agent comprising: water; tetravalent metal hydroxide particles; and an additive, wherein the additive contains at least one of a cationic polymer and a cationic polysaccharide. The present invention can provide a polishing agent which is capable of polishing an insulating film at a high speed with less polishing flaws, and having a high polishing rate ratio of a silicon oxide film and a stopper film, in the CMP technology of flattening insulating film. The present invention can also provide a polishing agent set for storing the polishing agent, and a method for polishing a substrate using this polishing agent.