Refinement of austenite grains by generating intra granular ferrite (IGF) in austenite grains is one of the most effective methods for improving the toughness of the heat-affected zone (HAZ). It is known that MnS precipitated on fine oxides enhance formation of IGF in austenite grains. Flux composed of CaO–Al2O3 is often used in secondary refining and Ca–Mn–Al complex oxide may form, and it is important to clarify the mechanism of MnS precipitation on oxide inclusions. Therefore, molten CaO–MnO–Al2O3 oxide was equilibrated with (Ca, Mn)S solid solution in Al2O3 crucible in an electric resistance furnace at 1648, 1698 and 1748 K in the present work. Phase relation between CaO·Al2O3 saturated molten CaO–MnO–Al2O3 oxide and (Ca, Mn)S solid solution was clarified.