PURPOSE: To provide the curing agent composition excellent in compatibility with various resins, cross-linking properties, smoothness, adhesion, impact resistance, etc., by mixing a specified prepolymer made from an isocyanate and a polyol, an oligomer of this isocyanate, and a blocking agent in a specified mixing ratio.

CONSTITUTION: The curing agent composition is prepared by mixing 50-80wt.% isocyanate-terminated prepolymer made from hexamethylene diisocyanate and a polyol of a number-average molecular weight of 300-5000, 5-30wt.% trimeric cyclooligomer of hexamethylene diisocyanate, and 10-30wt.% blocking agent. The coating composition is prepared by mixing the curing agent composition with an active-hydrogen-containing resin (e.g. unsaturated polyester resin or acrylic resin) in a molar ratio of the isocyanate groups of the curing agent to the active hydrogen atoms of the resin of 9:1 to 1:9.