To obtain an additive for detergents, excellent in compatibility with a polyvinyl pyrrolidone, or the like, and improved in thickening capability, and a detergent composition containing the same.

This additive for detergents uses a copolymer essentially comprising a constitutional unit represented by formula (1) and a constitutional unit represented by formula (2). In formula (1), R is hydrogen or methyl group, and (m) is an integer of 2-4. In formula (2), R1, R2, and R3 are each hydrogen atom, methyl group, or a (CH2)pCOOM2 group; M1 and M2 are each hydrogen atom, a monovalent metal, a bivalent metal, ammonium group, or an organic ammonium group; (p) is an integer of 0-2; and the (CH2)pCOOM2 group if in presence may be in the form of an anhydride.