PURPOSE: To improve impulse resistance by allowing the grains of one or more elements among Fe, Mo, W, and C to exist at specific area ratio on the surface of a base material made of Al alloy and forming the sulfides and nitrides of the above grains on the surface of the grains.

CONSTITUTION: For example, a base material composed of Al-Mg alloy is melted, and the grains of Fe, Mo, W, and C are added to the resulting molten alloy. The blending amount in this case is regulated so that the area ratio of these additive elements at the time when they are unevenly distributed in the surface of the base material becomes 10-70%. Then, the molten alloy is subjected, in the case where the additive elements are Fe, Mo, and W heavier than Al, to centrifugal casting and subjected, in the case of C lighter than Al, to gravity casting, by which the grains 7 of one or more additive elements, such as Fe, can be unevenly distributed in the surface of the base material 6 made of Al alloy. Subsequently, this base material made of Al alloy is heated in an atmosphere containing 5-70% ammonia and/or 10-500ppm H2S at 250-600°C for ≥0.5hr and subjected to nitrosulfurizing treatment. By this method, a layer 11 of the nitrides and/or sulfides, such as FeN and Fes, can be formed on the surface of the base material 6.