To enhance the coating film adhesiveness after forming a coating film containing pigment on an aluminum-made cap which is subjected to a very stringent forming and for which retort-resistance is required.

A polymer of an acrylic acid compound containing one or more carboxylic groups for each weight average molecular weight 1,000 is deposited with the deposit of 5 mg/m2 and 5,000 mg/m2 on an oxide film mainly consisting of Al and O in which the thickness is 200 nm; C content of the surface is 50 mg/m2, and the maximum concentration in the depth direction from the uppermost surface to an interface between the oxide film and aluminum is Mg:5 mass%, and H:10 mass%. In addition, a coating film containing pigment of 5 mass% to the dry weight is deposited on a base material in which a polymer of an acrylic acid compound containing one or more carboxylic groups for each weight average molecular weight 500 with the weight average molecular weight being 1,000 and 1,000,000 with the deposit of 5 mg/m2 and 5,000 mg/m2.