It is well known that the composition of inclusions is determined by alloying elements and by reaction with slag. For example, MgO·Al2O3 spinel-type inclusions form, even though Mg is not added, due to the supply of Mg through the reaction between slag and metal. To clarify the mechanism of compositional changes in inclusions, the authors have developed a kinetic model to simulate the reactions during the ladle refining process. In this study, experiments were conducted using an induction furnace, and the compositional changes in molten steel, slag, and inclusions were investigated. The inclusions were analyzed by P-SEM, which incorporates an automatic analysis system. By the application of the developed simulation model to these experiments, the validity of the model was evaluated. The inclusion composition gradually changed from Al2O3 to MgO·Al2O3 after the addition of Al, and the inclusions originating from slag were also observed at all times. The compositional change of the deoxidation product by the model calculation corresponded well to the observed variation in the composition of inclusions, and the calculated composition of inclusions originating from slag also agreed with the experimental results. The rate of compositional change increased with increasing Ar gas flow rate, and this tendency was captured well by the model. Therefore, the validity of the developed model is considered to be confirmed.