PURPOSE: To improve the spot weldability and chemical convertibility of an aluminum sheet or an aluminum alloy sheet by subjecting a plated film for chemical conversion treatment formed on the surface of an aluminum series metallic sheet to heating treatment and thereafter furthermore forming an oxidized film.

CONSTITUTION: For example, electroplating or the like is applied to the surface of an aluminum sheet or an aluminum alloy sheet to form a plated film for chemical conversion treatment. Next, the aluminum (alloy) sheet after the above plating is heated, e.g. to 200 to 500°C, and after that, an oxidized film is formed. This oxidized film is formed by bringing the sheet into contact with a mixture of air and steam or steam, bringing it into contact with a soln. of zinc nitrate or subjecting it to electrolysis treatment in an aq. soln.