The formation of MgO·Al₂O₃ spinel-type inclusions has often been reported even when Mg is not added during treatment. Many researchers have investigated the dissolution behavior of Mg from slag; however, studies on the reaction between molten steel and MgO-type refractory are limited. In this paper, the inclusion composition of Al-deoxidized steel melted in a MgO crucible, and mass transfer rates of Mg from a MgO rod and MgO in slag to Al-deoxidized molten steel were investigated. These studies clarified that Mg dissolved in the molten steel and the spinel formed not only with the reaction of molten steel and slag but also with the reaction of molten steel and MgO crucible. The dissolution rate of Mg from MgO rod increased as the rotation rate and Al content in steel increased. The MgO·Al₂O₃ spinel layer formed at the interface between the metal and MgO rod. The Mg content was higher for the reaction between molten steel and MgO in slag compared to the reaction between molten steel and MgO rods, as it equilibrated with MgO activity in slag.