A polishing agent comprises: a fluid medium; an abrasive grain containing a hydroxide of a tetravalent metal element; a first additive; a second additive; and a third additive, wherein: the first additive is at least one selected from the group consisting of a compound having a polyoxyalkylene chain and a vinyl alcohol polymer; the second additive is a cationic polymer; and the third additive is an amino group-containing sulfonic acid compound.