During copper smelting using a flash furnace, the solid magnetite (Fe3O4) phase that stagnates at the slag/matte interface inhibits the absorption of the suspended matte in the slag into the matte, resulting in copper loss. However, the phenomena that occur at the magnetite/matte interface are complex, and the effect of gas formation at the interface on the removal of magnetite has not been studied. In this study, we elucidated the effect of gas formation on the dissolution of the magnetite phase into the matte by directly observing the high-temperature reaction interface through the magnetite thin film. In contrast to the rapid dissolution of magnetite into FeS in the absence of gas formation, magnetite dissolution was strongly inhibited at the Cu2S/magnetite interface by the generation, agglomeration, and accumulation of SO2 gas bubbles. A quantitative analysis of the dissolution rate of the magnetite phase into Cu2S indicated that the mass transfer rate of Fe in the matte is extremely low. We also discuss the contribution of the generated SO2 gas to the inhibition of the interfacial reaction.