MgO–C refractory is a widely employed refractory in steel refining and it is considered as a Mg source to form MgO·Al₂O₃ spinel inclusions in steel melt. In this study, MgO–C refractories with various carbon contents were immersed into Al-killed molten steel to investigate the dissolution behavior of Mg from MgO–C refractory. The result showed that Mg gradually dissolved into the steel melt from the refractory, and spinel inclusions were formed. Al in the steel melt reduced MgO in the refractory and consequently, Mg was dissolved into the steel melt. The dissolved Mg increased with the Al content in the steel melt. Moreover, C in the MgO–C refractory also reduced MgO in the refractory and supplied Mg to the steel melt. However, when the C concentration in the refractory was lower than 10 mass%, Mg was not supplied. The reduction in MgO by Al and C occurred independently, and the Mg content in the steel was the sum of the Mg supplied by these reactions. In some regions at the refractory–steel melt interface, a spinel layer was observed; however, this layer was not formed uniformly.