In-situ observation of carburization and melting behavior of sulfur saturated iron by CO gas and carbon concentration measurement were carried out to discuss the effect of sulfur. The time for starting the melt formation on the sulfur saturated iron is twice longer than that on iron without sulfur because the average carburization rate of former iron is very slow compared with that of latter one. On the other hand, the time from reaching the saturated carbon concentration to starting the melt formation is short. The reason may be that the carburization rate is large when the carbon concentration reaches to the saturated one and the critical energy for the melt formation is smaller. The carburization to sulfur saturated iron is mixed control of desorption reaction of oxygen and sulfur on the iron surface and diffusion of carbon and sulfur into iron.