It is an object of the present invention to provide a detergent builder comprising a copolymer excellent in compatibility with a liquid detergent and capable of exhibiting excellent basic abilities such as prevention of recontamination and detergency ratio, and a detergent composition containing the detergent builder. The above-mentioned detergent builder comprises a copolymer obtained by copolymerization of monomer components containing a polyalkyleneimine unsaturated monomer and other monomers.