Provided is a novel copolymer capable of exhibiting superior ability to capture calcium ions. The copolymer according to the present invention comprises 0.1-99 mol% of a structural unit (A) derived from a monomer (a) which is a monoethylenically unsaturated monocarboxylic acid (salt), 0.1-99 mol% of a structural unit (B) derived from a monomer (b) which is a monoethylenically unsaturated dicarboxylic acid (salt) or an anhydride thereof, and 0.1-50 mol% of a structural unit (C) derived from an ether bond-containing hydrophobic monomer (c).
