To obtain a chelating agent (composition) having excellent chelating performances especially for light metal ions and a coagulation and hardening retarder for gypsum, a detergent composition, a scale inhibitor and a liquid agent for plating (including an electrolytic stripping agent for metals) using the chelating agent.

This chelating agent composition is characterized as comprising an amidopolycarboxylic acid in the form in which one hydrogen atom of NH2 or NH produced by reacting the NH2 group of ethylenediamine or the NH site of a polyethyleneimine having 2-1,000 polymerization degree with an α,β- dicarboxylic acid is substituted with a monovalent acid group derived from the α,β-dicarboxylic acid and/or its salt as an active ingredient.