A polishing agent for chemomechanically polishing a base having a carbon-based material and an insulating material to remove at least a part of the carbon-based material, the carbon-based material having a carbon content of 60 to 95 atm % as measured by X-ray photoelectron spectroscopy, the polishing agent comprising: an abrasive grain comprising silica; an allylamine-based polymer; and water, wherein a mass ratio of a content of the allylamine-based polymer with respect to a content of the abrasive grain is 0.002 to 0.400, and the abrasive grain has a positive charge in the polishing agent.