A method of treating a metallic surface which comprises treating a metal-coated steel panel with a nonchromate metallic surface treating agent containing, in each liter thereof, (a-1) a silane coupling agent and/or a hydrolytic condensation product thereof, (a-2) water-dispersible silica in a proportion, and (a-3) a zirconium compound and/or a titanium compound, drying the treated steel panel and coating it with an anticorrosion coating agent containing, in each liter of an aqueous resin solution or dispersion, (b-1) a silane coupling agent and/or a hydrolytic condensation product thereof, (b-2) water-dispersible silica and (b-3) at least one phosphorus-containing ion selected from among phosphate ion, phosphite ion and hypophosphite ion.