Abstract: A zincate-treated article of an Al-Mg-Si base alloy has a zincate coating film having a Zn crystal grain size of 1.0 .mu.m or smaller on an Al-Mg-Si base alloy body having a Cu content of 0.1 to 1.5 weight %. The Zn crystal grain size is preferably equal to or smaller than 0.95 .mu.m. This zincate-treated article can be manufactured by treating an Al-Mg-Si base alloy body having a Cu content of 0.1 to 1.5 weight % in a zincating treatment bath containing 100 to 300 g/l of sodium hydroxide, 5 to 20 g/l of zinc oxide, 2 to 10 g/l of iron chloride, 5 to 20 g/l of Rochelle salt and 50 to 200 ml/l of water glass at a temperature of 20.degree. to 80.degree. C. for 5 to 60 seconds. By adjusting the Zn crystal grain size in the zincating treatment in this manner, a zincate-treated article of an Al-Mg-Si base alloy having an excellent adhesion can be acquired even in a single stage zincating treatment.