vA water-base coating composition that is suitable for use in marine vessels, bridges, etc., excelling in luster, coating film appearance and corrosion resistance, and that even when applied under high-humidity conditions, is free from deterioration of these properties; and a coated steel sheet having the above water-base coating composition applied thereto. There is provided a water-base coatingcomposition containing a water-dispersed vinyl modified epoxy ester resin and calcium carbonate, characterized in that the calcium carbonate is precipitated calcium carbonate having been treated witha salt of carboxylated vinyl polymer and contained in a ratio of 20 to 50 vol.% based on the solid contents of the coating composition, and that the volume ratio of coating composition pigment/whole solid contents is 50% or below.