Soil contamination and accompanied groundwater contamination by arsenic is a serious environmental problem worldwide. In this study, a series of experiments were performed to obtain the information on the influence of soil properties (arsenate content, iron oxyhydroxide content, and alkali buffer capacity) on the arsenate immobilization by magnesium oxide (MgO), with the final objective of deepening the understanding of the arsenate immobilization mechanisms. Experimental results showed that higher amount of MgO was required to immobilize arsenate below 10 μgAs/L (the permissible level for leaching tests in Japan) for the soils having a higher arsenate content or lower iron oxyhydroxide content. Experimental data also indicated that low alkali buffer capacity positively influences arsenate immobilization while high alkali buffer capacity negatively influences arsenate immobilization. These results clearly showed that the soil properties investigated in this study significantly influences arsenate immobilization and that the MgO dosage needs to be carefully determined depending on these soil properties.