Aluminium alloy substrate used for magnetic disc, and method of manufacturing the same

The present invention provides an aluminium alloy substrate used for a magnetic disc, and a method of manufacturing the same. The aluminium alloy substrate is resistant to impact, has great flatness, and has a plated surface with excellent smoothness. The aluminium alloy substrate is characterized by comprising the following aluminium alloys: below 0.03 by mass of Si, below 0.03 by mass of Fe, over 3.5 and below 4.5 by mass of Mg, below 0.2 by mass of Cr, and at least one selected from over 0.01 and below 0.20 by mass of Cu and over 0.01 and below 0.40 by mass of Zn, the balance being Al and unavoidable impurities, wherein the degree of flatness is less than 5 [mu]m, and the area rate of equiaxed grains on the surface of the substrate is less than 30%.