This paper investigated the effects of anodizing time on the formation of anodic oxide films on a Mg-Al alloy in alkaline solution. The thickness of the anodic oxide film was increased by increasing the time required to generate the active dissolution reaction. When anodizing at various anodizing time, the potential after passivity increased with time, which implies growth in film thickness. When the anodizing time was varied, the quantity of oxygen increased with time in the white areas (the film), i.e., more film was observed in the SEM and EDX analyses.