An aluminum alloy containing Si: 1.5-12% (mass % here and hereinafter), Mg: 0.5-6% and, optionally, at least one of Mn: 0.5-2%, Cu: 0.15-3% and Cr: 0.04-0.35% and, further, containing Ti: 0.01-0.1% and the balance of Al and inevitable impurities, in which the average grain size of crystallized grains of Si system compounds is from 2 to 20 .mu.m and an area ratio thereof is from 2 to 12%. The alloy is melted to obtain a cast ingot having DAS (Dendrite Arm Spacing) of 10 to 50 .mu.m, which is then put to a soaking treatment at 450 to 520.degree. C. and then to extrusion molding. The aluminum alloy has excellent machinability with no addition of low melting metals.