To obtain a coating composition which is excellent in curability (particularly low-temperature curability) of coating films and can form coating films excellent in chipping resistance, adhesive properties, and the like, and to provide a method for forming a double-layered coating film.

There are provided a water-based heat-curable coating composition comprising a hydroxylated resin, a crosslinking agent and a phosphatic resin, wherein the phosphatic resin is a polymer formed from a monomer mixture comprising 20-30 wt.% of a phosphatic monomer, 20-30 wt.% of a hydroxylated monomer and 40-60 wt.% of methyl methacrylate, as constituent components, and a method for forming a double-layered coating film comprising coating an object with the water-based heat-curable coating composition and then with an organic-solvent-type clear coating material, and heating the resulting object to cure both the coating films at the same time.