PURPOSE: To form a surface film not being destroyed even if being exposed to a high temp. by controlling its immersing time into a chromic acid treating soln. into a certain range.

CONSTITUTION: The immersing time into chromic acid of an aluminum alloy of the alloy number 2021 or the like in JIS-H-4040 is regulated to 165-200sec, and it is regulated to 165-200sec as for the alloy number 6061. The chromic acid treatment is executed after preliminary treatment such as degreasing, washing and neutralizing, as for the treating soln., the conventionally well-known one is used, and it may be executed by an ordinary method. By this method, the surface film not being easily destroyed even if being exposed to a high temp. of ≥60°C and excellent in corrosion resistance can be formed.