PURPOSE: To provide a method for phosphating a metallic surface by which a high quality zinc phosphate coating film excellent in corrosion resistance can be formed on an Al alloy member or a combined member consisting of an Al alloy member and a steel member.

CONSTITUTION: The concn. of F ions and that of hydrosilicofluoric acid in a zinc phosphate forming chemical treating soln. for treating the surface of an Al alloy member or a combined member consisting of an Al alloy member and a steel member are regulated to 100-200ppm and 750-1,000ppm, respectively, and the free acidity of the soln. is regulated to 0.5-0.8 point. Since the concn. of F ions is reduced the amt. of Na incorporated into a zinc phosphate coating film is reduced and the corrosion resistance is improved. Since the concn. of hydrosilicofluoric acid is increased, the formation of a zinc phosphate coating film is accelerated.