PURPOSE: To form polishing layer to seal the cracks by subjecting the surface crack grooves of a plating film to ultra-finish polishing.

CONSTITUTION: The polishing amount for ultra-finish working is varied depending upon the groove width state of each crack opening in a plated film 5 and the crack density thereof to form a polishing layer 3 on the surface of the plated film 5, thereby sealing the crack openings. Namely, the ultra-finish polishing which is based on the (f) value defined in accordance with the crack density is performed. As a result, the plated film 5 ceases to have any permeation crack via which corrosion substances permeate the surface of a base member. This enables special increase in the corrosion resistance of the iron base member covered with the hard chrome plated film 5.