Water-base coating composition

A non-chromium water-base coating composition which can form a coating film excellent in chemical resistance, corrosion resistance of flat areas of metal sheets, corrosion resistance of worked areas or end faces of metal sheets, and adhesion; and coated metal sheets provided with coating films excellent in these properties. A water-base coating composition comprising a cationic resin (A), a polyisocyanate curing agent (B), and a rust-preventive pigment mixture (C), characterized in that the rust-preventive pigment mixture (C) has the following composition: Rust-preventive pigment mixture (C): a composition comprising at least one vanadium compound (c1) selected from among divanadium pentaoxide, calcium vanadate and ammonium metavanadate, a silicon compound (c2), and a calcium salt (c3) of a phosphoric acid, wherein the amounts of the vanadium compound (c1), the silicon compound (c2), the calcium salt (c3), and the rust-preventive pigment mixture (C) are 1 to 30 parts by mass, 1 to 30 parts by mass, 1 to 30 pa