A chromate-treated steel strip comprising a chromate film on a zinc plated steel strip is improved in corrosion resistance and coating adherence by forming the chromate film of three layer structure consisting of a metallic Cr layer, a Cr.sup.3+ oxide layer, and an outermost surface layer of SiO.sub.2, and Cr.sup.3+ oxide plus Cr.sup.6+ oxide. The three layered chromate film is deposited on a zinc plated steel strip by effecting cathodic electrolysis in a bath containing hexavalent chromium, colloidal silica, optional alumina sol, and a fluoride, by supplying electricity at a specific current density to a controlled quantity.