PROBLEM TO BE SOLVED: To provide a copolymer yielding a cosmetic composition and a conditioner each exhibiting, without adversely affecting in-use tactile properties such as smoothness, etc., excellent conditioning effects such as post-dry non-stiffness and finger combing friendliness.SOLUTION: The provided copolymer includes a constituent unit derived from a monomer (a) and at least one of constituent units derived respectively from monomers (b) through (d). The copolymer includes, with respect to the total mass thereof, 1-99 mass% of the constituent unit derived from the monomer (a) and 50-100 mass% of a constituent unit derived from a vinyl monomer (A) possessing carboxyl groups, whereas the mass average molecular weight thereof is 3,000-400,000 monomer (a): acrylic acid and/or methacrylic acid monomer (b): CH=C(R)-CO-(O-(CH)-CO)-OH monomer (c): CH=C(R)-CO-Y-(Z-O)-R monomer (d): CH=C(R)-COO-R.
