To obtain an Al-Mg-Si aluminum alloy sheet small in cold aging properties and excellent in baking hardenability and formability.

An Al-Mg-Si aluminum alloy contg., by weight, 0.3 to 1.0% Mg and 0.8 to 1.3% Si is subjected to melting and casting, is thereafter subjected to soaking treatment and is next subjected to hot rolling and cold rolling to control its sheet thickness to a desired one, which is subsequently subjected to solution heat treatment of executing heating at 510 to 590°C and holding it to the temp. range for ≤60 sec as final heat treatment, is thereafter cooled to 400°C at a cooling rate of ≥20°C/sec, is subsequently cooled in the temp. range of 400 to 300°C at a rate of 4 to 10°C/sec and is then cooled to room temp. at a rate of ≥4°C/sec.