Carburization and melting processes of solid Fe under the co-existence of graphite and wüstite has been investigated based on a confocal laser scanning microscope “in-situ” observation of the movement of the interface between Fe–C melt and solid Fe at 1523 K. In the experiments, strong stirring flow at the surface of the Fe–C melt is observed. This flow is found to be Marangoni flow and is introduced by the surface tension gradient due to the difference of oxygen concentration between the Fe–C melt interface with graphite and solid iron. The velocity of Marangoni flow is measured from the moving velocities of bubbles at the molten Fe–C surface. The melting rate of Fe is analyzed by computational fluid flow simulations of the Fe–C melt. Based on these results, it is confirmed that the carburization and melting rate of solid Fe can be enhanced by allowing Fe–C melt to come into contact with wüstite and graphite simultaneously.