The present invention relates to an aluminum alloy for a magnetic disk substrate having a composition consisting of, by percent by weight, 2.0 to 6.0% of Mg, 0.05 to 0.15% of Cu, 0.10 to 0.30% of Zn and 0.05 to 0.12% of Zr, wherein the contents of Cu, Zn and Zr satisfy the relationship: 0.15%.ltoreq.2Cu+6Zr-3Zn.ltoreq.0.32% (wherein Cu, Zr, and Zn respectively represent their contents in terms of percent by weight), and the composition further consists of one or both of 0.01 or more to less than 0.05% of Cr and 0.01 or more to less than 0.05% of Mn, impurities consisting of Si, Fe and Ti, which are respectively regulated to be 0.05% or less, 0.05% or less, and 0.02% or less, and other inevitable impurity elements respectively regulated to be 0.02% or less, with the remainder being Al. The present invention also relates to a clad material for a magnetic disk substrate, in which one or both surfaces of a core material, consisting of at least less than 3.0% of Zn with the remainder being Al and inevitable impurity elements, are clad with a surface material consisting of the aluminum alloy having the above composition. The above aluminum alloy composition makes it possible to manufacture a large storage memory capacity magnetic disk in which there is much less formation of micropits or nodules.