PURPOSE: To remove an oxide film easily and effectively without damaging the plating quality of a steel plate almost perfectly, in removing the oxide film electrolytically, by providing anodes respectively opposed to both surfaces of a plated steel plate in an electrolyte.

CONSTITUTION: An one-side alloied molten zinc plated steel plate 6 after alloying treatment is negatively charged by a conductor roll 10 and immersed in an electrolyte 8 to be passed between anodes 9, 9 toward an direction. During this time, the oxide film comprising various iron oxides adhered to the non-plated surface 61 thereof is electrolytically removed. As the electrolyte 8, a conventional sulfuric acid solution is used and an inhibitor for suppressing the elution of zinc is added thereto. By electrolysis between the plated surface of the plate 6 and the anode 9 opposed thereto, zinc in the liquid 8 is precipitated onto the plated surface at first to form a zinc skin film and the plated surface is protected by said skin film. By this skin film, the elution of zinc is also prevented and the elution preventing effect is further increased by the inhibitor.