A brake piston having required mechanical strength is produced through fewer steps than in conventional processes. An aluminum alloy containing Si and Mg, wherein the aluminum alloy has a Si concentration (CSi, mass %) and a Mg concentration (CMg mass %) falling within a range surrounded by four points A (0.75, 1.25), B (1.4, 1), C (1.4, 0.6), and D (0.75, 0.85) in terms of a relationship thereof (CSi, CMg), and further has a Cu concentration of 0.07 to 0.9 mass %, a Mn concentration of 0.1 to 0.9 mass %, a Ti concentration of 0.005 to 0.15 mass %, a Cr concentration of 0.2 mass % or less, and a Fe concentration of 0.5 mass or less %, the balance being A1 and inevitable impurities is cast to obtain a rod-shaped ingot. The rod-shaped ingot, without being subjected to a homogenization treatment, is straightened within 3 (three) days after the casting. The straightened rod-shaped ingot is cut into a given thickness to obtain a forging raw material. This forging raw material, without being subjected to annealing, is cold-forged at a reduction rate of 25 to 90% within 7 (seven) days after the casting of the rod-shaped ingot into a cup shape, which is subjected to age-hardening without being subjected to a solution heat treatment.