To more improve the corrosion resistance of an aluminum alloy against sulfuric acid than prior art.

An aluminum alloy 3 is targeted which contains, by weight, 4-24% silicon, ≤5% copper, ≤1.5% magnesium, ≤1.3% iron, and the balance aluminum with inevitable impurities. The aluminum alloy 3 is immersed in a high-concentration sulfuric acid aqueous solution 2 with its concentration exceeding 60% after removing any oxide film and oil on its surface. After an anticorrosion layer 4 by corrosion products is formed on a surface of the aluminum alloy 3, the aluminum alloy 3 is pulled up from the high-concentration sulfuric acid aqueous solution 2. The aluminum alloy 3 is cleaned with water so as not to remove the anticorrosion layer 4, and then dried.