PURPOSE: To impart excellent resistance to blackening and corrosion and coating film adhesion by cathodizing a galvanized steel sheet in an electrolyte contg. a specified metal and then chromating the sheet.

CONSTITUTION: A galvanized steel sheet is electrolyzed in an electrolyte contg. one or more kinds of metals among Ni, Co and Fe with the sheet as a cathode to deposit the metals on the sheet surface at 1-50μm/m2 and then chromated. Nickel sulfate, nickel chloride, nickel nitrate, cobalt sulfate, cobalt chloride, iron chloride and iron sulfate are used to prepare the electrolyte, and the electrolyte is preferably controlled to ≥pH1.0 with sulfuric acid, hydrochloric acid, phosphoric acid, etc. Further, the cathode current density is preferably adjusted to ≥2A/dm2 in electrolysis.