Disclosed is a coated steel sheet which can keep corrosion resistance even under strongly alkaline corrosive environments and has excellent corrosion resistance and alkali resistance. The coated steel sheet comprises a steel sheet, a chemical-conversion-treated coating film formed on the surface of the steel sheet, a primer coating film formed on the surface of the chemical-conversion-treated coating film, and a top coating film formed on the surface of the primer coating film. The chemical-conversion-treated coating film comprises a fluoride of titanium or a fluoride of zirconium, an oxide or hydroxide of titanium, and an oxide or hydroxide of zirconium. The primer coating film comprises an organic resin, an anti-corrosive pigment comprising one or two compounds independently selected from the group consisting of magnesium phosphate and zirconium phosphate, and barium sulfate.