A thermosetting water-borne coating composition which, when used as a water-borne coating, have both sufficient curability and sufficient storage stability and provides coating films excellent in water resistance, a method of forming a coating film using the same and a method of forming a multilayer coating film using the same are provided. The coating composition comprises a polycarbodiimide compound modified for hydrophilicity and a carboxyl-containing aqueous resin composition, wherein the polycarbodiimide compound has a structure such that a carbodiimide unit and a polyol unit occur alternately via a urethane bond and repeatedly and a hydrophilic unit occurs at both molecular termini and is bound to a carbodiimide unit via a urethane bond.