PURPOSE:To prevent the generation of a recessed flaw on a stop-off surface in the stage of plating a steel strip formed with a layer of a plating stop-off agent on one surface by removing the metallic balls sticking on the stop-off surface before the stop-off agent surface contacts with a roll. CONSTITUTION:A steel strip 1 is coated with a plating stop-off agent on one surface A thereof with a coater 2 and is then subjected to calcination of the stop-off agent,that is, the formation of a stop-off agent layer together with annealing in a continuous annealing furnace 3. The strip 1 is thereafter dipped in a molten metal pot 4, and is subjected to hot dipping on one side, and is guided by rolls 5-8 so as to be transferred to the succeeding stage. A brushing device 9 is provided in a desired position in the section after the passage of the port 4 and before the contact of the surface A with the roll 8, for example, in the position between the rolls 5 and 6, to brush the surface A and to remove the fine metallic balls sticking on the surface A. The formation of a recessed flaw on the surface A owing to the contact of said surface having such balls sticking thereon with the roll 8 is thus prevented.