PROBLEM TO BE SOLVED: To obtain a water-soluble copolymer useful for detergent composition or as a water-treating agent or the like, which copolymer comprises structural units derived from a monoethylenically unsaturated dicarboxylic acid monomer, structural units derived from a monoethylenically unsaturated monocarboxylic acid monomer, and structural units derived from a sulfo-containing monoethylenically unsaturated monomer and has specified functions such as a Ca ion scavenging capacity. SOLUTION: This copolymer essentially consists of structural units derived from (A) a 4-6C monoethylenically unsaturated dicarboxylic acid (salt or anhydride) monomer, structural units derived from (B) a 3-8C monoethylenically unsaturated monocarboxylic acid (salt) monomer, and (C) structural units derived from a sulfo(sulfonate)-containing monoethylenically unsaturated monomer and has a Ca ion scavenging capacity of at feast 0.40, a capacity of dispersing clay in high-hardness water of at least 0.35, and a gelation resistance of at least 0.35. The copolymer is one prepared by polymerizing 30-70 mol% component A with 30-70 mol% component B, 2-16 mol% component C, and 0-5 mol% copolymerizable monoethylenically unsaturated monomer and having a weight-average molecular weight of 2,000-20,000.