A method for forming a conductive film such as silver, copper, tin or the like on a passivating light metal selected from aluminum, magnesium and titanium through a base film, wherein the base film is a nickel-phosphorus film, and the base film is formed of a predetermined solubility It is formed using an electric nickel-phosphorus plating bath containing nickel salt, a compound containing phosphorus, a complexing agent, a surfactant, a buffer, and a brightener, and the underlying film, or the underlying film and the conductive film, is formed in a low temperature region of 30°C or higher. By heat-treating under the conditions including, compared to the method without heat treatment, the adhesion of the underlying film to the passivation light metal can be strengthened, and the conductive film can be formed more strongly on the passivation light metal.
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