Disclosed is an aluminum alloy sheet having a chemical composition of an Si-containing Al--Mg--CU alloy. The aluminum alloy sheet exhibits a streak-shaped modulated structure at a diffraction grating points of an Al--Cu--Mg--system compound in the electron beam diffraction grating image. The above mentioned streak can be generated efficiently when the alloy essentially consists of 1.5 to 3.5% by weight of Mg, 0.3 to 1.0% by weight of Cu, 0.05 to 0.6% by weight of Si, and the balance of Al and inevitable impurities, and the ratio of Mg/Cu is in the range of 2 to 7. The alloy contains 0.01-0.50% of at least one element selected from the group consisting of Sn, Cd, and In.