To provide: an additive for hair cosmetic which surmounts defects of an additive for a semisynthetic hair cosmetic represented by polyquaternium-10 or defects of an additive for a synthetic hair cosmetic represented by polyquaternium-6 or -7 and which gives flexible feeling to finished hair; and a hair cosmetic containing the additive.

The additive for hair cosmetic is produced by dissolving a natural polymer and/or a natural polymer derivative, both having OH groups, and then graft-copolymerizing a cationic water-soluble monomer and a nonionic water-soluble monomer in an aqueous medium. The additive is added to a hair cosmetic composition.