To provide a new copolymer improving foam quality in cleansing the hair using the cleanser therefrom and hair strokability with fingers in rinsing the hair and imparting moist touch feeling and conditioning effect to the hair and the skin after use.

The copolymer is provided, which is obtained by copolymerization between a specific cationic group-containing monomer, a diallylamine-based monomer and a (meth)acrylamide-based monomer. By using this copolymer, a cosmetic significantly improved in moist touch feeling and conditioning effect is obtained. When this copolymer is used in a hair/skin cleanser, it has been found that the cleanser is good in foamability.