In a throttle body (10), a passive film (14) and a corrosion-resistant film (16) are stacked in that order on a substrate (12) of aluminum. The passive film (14) has a concave part (18) on its corrosion-resistant film (16) side. A metal layer (20) is provided in the concave part (18). In shot blasting, upon collision of a blasting material against the corrosion-resistant film (16), a part of the blasting material is separated and adhered onto the resultant concaves to form the metal layer (20). For example, aluminum having a purity of not less than 98% is selected as the blasting material.