The diffusion behavior of each element at the interface has to be clarified to comprehensively understand the reaction between oxide inclusions and solid steel at heat treatment temperatures. To this end, we performed diffusion couple experiments to investigate the interaction between Fe–Mn–Si alloy and MnO–SiO₂–FeO oxide. As the first step, we investigated the composition of the oxide equilibrated with the molten steel deoxidized by Mn–Si. Results indicate that the equilibrium oxide was not a binary compound of MnO and SiO₂ but a ternary compound of MnO, SiO₂, and FeO. In diffusion couple experiments, fine oxide particles and fine metal particles were observed near the interface in the alloy and in the bulk oxide, respectively, after heat treatment at 1673 K and 1473 K. This phenomenon indicated that the alloy and bulk oxide in the diffusion couple were not equilibrated at these temperatures, despite the equilibration of both phases at 1823 K. The precipitation behavior of the particles can be related to the diffusion of oxygen from the oxide to the alloy. The oxide particles were precipitated near the interface, and the Mn content in the alloy decreased in this region owing to the consumption of Mn for particle formation. The formation of metal particles and the decrease of FeO content in oxide indicate the reduction of unstable FeO in oxide at these temperatures.