PURPOSE: To prevent the blackening phenomenon to be generated on the plated surface accurately, stably and easily by treating the plated surface of a zinc- plated steel sheet with an alkaline soln. contg. iron ions, aluminate ions and zincate ions.

CONSTITUTION: Chromate treatment is applied to the steel sheet which is hot- dipped with zinc or a zinc alloy to prevent the generation of white rust. The surface of the plated steel sheet is treated with an alkaline soln. contg. iron ions are aluminate ions and/or zincate ions for controlling the deposition of Fe on the surface. Either ferrous ions or ferric ions can be used as the iron ions and the iron ions are used along with a complexing agent. The pH of the alkaline soln. is regulated to about 9W14 and the weight ratio of the aluminate ions and/or zincate ions to Fe is controlled to about 0.1W10 when expressed in terms of Al and/or Zn. The treatment temp. is adjusted in the range from room temp. to about 70°C. The deposition quantity of iron necessary for preventing blackening is regulated to about 0.2W10mg/m2.