Al–Mg alloy is one of the most important industrial aluminum products used in can lids, panel, wheel, welding materials etc. It is known, however, that magnesium in the alloy enhances dross formation on the molten aluminum surface during melting process. Suppression of the dross formation is one of the most important tasks in aluminum industry. In the present work, in situ observation of dross formation during melting of Al–Mg alloy was conducted. By the air oxidation of molten Al–Mg alloy at 800 °C, horizontally projected area of dross phase formed on the surface of the melt and weight gain of the alloy were measured. It was observed that the projected area of dross phase drastically increased after an incubation period and then it changed a little. On the other hand, weight gain of the alloy continuously increased and it became faster with increasing magnesium content of the alloy. These findings help to clarify the dross formation mechanism.