The inclusion of the MgO·Al2O3 (MA) spinel and CaO–Al2O3 are occasionally observed during the refining of Al–killed steel, even without the intentional additions of Ca and Mg. Many studies have focused on the source of Mg and Ca; however, especially for the formation of CaO–Al2O3–type inclusions, some recent results showed that Ca was difficult to dissolve from refining slag, even when the Al content in molten steel was high. To confirm these differences, industrial experiments were designed in this study, and the effects of the FeO and MnO contents, as well as the impurities of the alloying materials, were discussed. The results showed that, when the FeO and MnO contents in slag were high (about 10 mass%) at the start of LF (ladle furnace), all inclusions remained as Al2O3, despite alloying. Using the slag with low FeO + MnO content (<1~2 mass%), the Al2O3 inclusions changed to the MA spinel, but CaO–Al2O3 inclusions were not observed, indicating that CaO–Al2O3 inclusions were difficult to form by the steel/slag reactions under the current conditions. Only for the molten steel that contained a low level of dissolved oxygen and a large amount of Fe–Si, which contained Ca as the impurity was added, CaO–Al2O3 inclusions were generated.