The present invention provides a curing agent comprising a polyisocyanate compound obtained from hexamethylene diisocyanate and containing not more than 0.5% by weight of hexamethylene diisocyanate monomer, 21 to 60% weight of uretidine dione dimer and 11 to 40% by weight of isocyanurate cyclic trimer, the uretidine dione dimer and the isocyanurate cyclic trimer being prepared from a product obtained by urethane modification of 0.5 to 15 mole % by mole of total isocyanate groups of the hexamethylene diisocyanate by a dihydric alcohol having at least two hydroxyl groups and a branched hydrocarbon group having 4 to 35 carbon atoms in the molecule, which the side chain hydrocarbon groups in the branched hydrocarbon group have 2 to 33 carbon atoms in total. The curing agent has a low viscosity and a good compatibility with a solvent, and can provides a polyurethane paint or adhesive composition having good curing property, drying property, adhesion strength and weathering resistance.