Superplasticity at relatively low temperatures around 573K has been investigated a severely rolled sheet of an Al-4.2mass%Mg alloy. It was already found that fine recrystallized grains of about 6 mum was gotten by annealing for 120s at 573K in a salt bath for the severely rolled sheet. The maximum elongation to failure of 208% is obtained at 563K and 1.4x10(-3)s(-1) at a higher heating rate of 5Ks(-1), though the sheet has a small thickness of only 0.1mm. It is suggested from analyses of stress and fracture surface that the elongation for 0.57Ks(-1) and 5Ks(-1) is attained by different mechanisms.