An aluminum-alloy sheet includes 0.10 to 0.40 mass % of Si, 0.35 to 0.80 mass % of Fe, 0.10 to 0.35 mass % of Cu, 0.20 to 0.80 mass % of Mn, and 1.5 to 2.5 mass % of Mg, the balance being Al and unavoidable impurities, wherein a content ratio (Si/Fe) of the Si to the Fe is 0.75 or less, the solute Mn content is 0.12 to 0.20 mass %, and the aluminum-alloy sheet has a proof stress of 225 N/mm2 or more after having been baked at 270° C. for 20 seconds.