The invention provides a means to prepare a highly water-resistant printed material by the ink-jet printing method despite the water-solubility of the dye in the aqueous ink used in the ink-jet printing method. The inventive method comprises: overcoating the surface of the sheet material, which has a water-absorptive surface layer with receptivity of the aqueous ink and printed by the ink-jet printing method, with a curable polyisocyanate compound and bringing the overcoating layer under a condition capable of curing the polyisocyanate compound.